Reinterpret dots in field names as object structure (#79922)

DocumentParser parses documents by following their object hierarchy, and
using a parallel hierarchy of ObjectMappers to work out how to map leaf fields.
Field names that contain dots complicate this, meaning that many methods
need to reverse-engineer the object hierarchy to check that the current parent
object mapper is the correct one; this is particularly complex when objects
are being created dynamically.

To simplify this logic, this commit introduces a DotExpandingXContentParser,
which wraps another XContentParser and re-interprets any field name containing
dots as a series of objects. So for example, `"foo.bar.baz":{ ... }` is represented
as `"foo":{"bar":{"baz":{...}}}`. DocumentParser uses this to automatically 
expand all field names containing dots when parsing the source.

libs/x-content/src/main/java/org/elasticsearch/xcontent/DelegatingXContentParser.java

@@ -0,0 +1,244 @@
+/*
+ * Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
+ * or more contributor license agreements. Licensed under the Elastic License
+ * 2.0 and the Server Side Public License, v 1; you may not use this file except
+ * in compliance with, at your election, the Elastic License 2.0 or the Server
+ * Side Public License, v 1.
+ */
+
+package org.elasticsearch.xcontent;
+
+import org.elasticsearch.core.CheckedFunction;
+import org.elasticsearch.core.RestApiVersion;
+
+import java.io.IOException;
+import java.nio.CharBuffer;
+import java.util.List;
+import java.util.Map;
+import java.util.function.Supplier;
+
+public abstract class DelegatingXContentParser implements XContentParser {
+
+    protected abstract XContentParser delegate();
+
+    @Override
+    public XContentType contentType() {
+        return delegate().contentType();
+    }
+
+    @Override
+    public void allowDuplicateKeys(boolean allowDuplicateKeys) {
+        delegate().allowDuplicateKeys(allowDuplicateKeys);
+    }
+
+    @Override
+    public Token nextToken() throws IOException {
+        return delegate().nextToken();
+    }
+
+    @Override
+    public void skipChildren() throws IOException {
+        delegate().skipChildren();
+    }
+
+    @Override
+    public Token currentToken() {
+        return delegate().currentToken();
+    }
+
+    @Override
+    public String currentName() throws IOException {
+        return delegate().currentName();
+    }
+
+    @Override
+    public Map<String, Object> map() throws IOException {
+        return delegate().map();
+    }
+
+    @Override
+    public Map<String, Object> mapOrdered() throws IOException {
+        return delegate().mapOrdered();
+    }
+
+    @Override
+    public Map<String, String> mapStrings() throws IOException {
+        return delegate().mapStrings();
+    }
+
+    @Override
+    public <T> Map<String, T> map(Supplier<Map<String, T>> mapFactory, CheckedFunction<XContentParser, T, IOException> mapValueParser)
+        throws IOException {
+        return delegate().map(mapFactory, mapValueParser);
+    }
+
+    @Override
+    public List<Object> list() throws IOException {
+        return delegate().list();
+    }
+
+    @Override
+    public List<Object> listOrderedMap() throws IOException {
+        return delegate().listOrderedMap();
+    }
+
+    @Override
+    public String text() throws IOException {
+        return delegate().text();
+    }
+
+    @Override
+    public String textOrNull() throws IOException {
+        return delegate().textOrNull();
+    }
+
+    @Override
+    public CharBuffer charBufferOrNull() throws IOException {
+        return delegate().charBufferOrNull();
+    }
+
+    @Override
+    public CharBuffer charBuffer() throws IOException {
+        return delegate().charBuffer();
+    }
+
+    @Override
+    public Object objectText() throws IOException {
+        return delegate().objectText();
+    }
+
+    @Override
+    public Object objectBytes() throws IOException {
+        return delegate().objectBytes();
+    }
+
+    @Override
+    public boolean hasTextCharacters() {
+        return delegate().hasTextCharacters();
+    }
+
+    @Override
+    public char[] textCharacters() throws IOException {
+        return delegate().textCharacters();
+    }
+
+    @Override
+    public int textLength() throws IOException {
+        return delegate().textLength();
+    }
+
+    @Override
+    public int textOffset() throws IOException {
+        return delegate().textOffset();
+    }
+
+    @Override
+    public Number numberValue() throws IOException {
+        return delegate().numberValue();
+    }
+
+    @Override
+    public NumberType numberType() throws IOException {
+        return delegate().numberType();
+    }
+
+    @Override
+    public short shortValue(boolean coerce) throws IOException {
+        return delegate().shortValue(coerce);
+    }
+
+    @Override
+    public int intValue(boolean coerce) throws IOException {
+        return delegate().intValue(coerce);
+    }
+
+    @Override
+    public long longValue(boolean coerce) throws IOException {
+        return delegate().longValue(coerce);
+    }
+
+    @Override
+    public float floatValue(boolean coerce) throws IOException {
+        return delegate().floatValue(coerce);
+    }
+
+    @Override
+    public double doubleValue(boolean coerce) throws IOException {
+        return delegate().doubleValue(coerce);
+    }
+
+    @Override
+    public short shortValue() throws IOException {
+        return delegate().shortValue();
+    }
+
+    @Override
+    public int intValue() throws IOException {
+        return delegate().intValue();
+    }
+
+    @Override
+    public long longValue() throws IOException {
+        return delegate().longValue();
+    }
+
+    @Override
+    public float floatValue() throws IOException {
+        return delegate().floatValue();
+    }
+
+    @Override
+    public double doubleValue() throws IOException {
+        return delegate().doubleValue();
+    }
+
+    @Override
+    public boolean isBooleanValue() throws IOException {
+        return delegate().isBooleanValue();
+    }
+
+    @Override
+    public boolean booleanValue() throws IOException {
+        return delegate().booleanValue();
+    }
+
+    @Override
+    public byte[] binaryValue() throws IOException {
+        return delegate().binaryValue();
+    }
+
+    @Override
+    public XContentLocation getTokenLocation() {
+        return delegate().getTokenLocation();
+    }
+
+    @Override
+    public <T> T namedObject(Class<T> categoryClass, String name, Object context) throws IOException {
+        return delegate().namedObject(categoryClass, name, context);
+    }
+
+    @Override
+    public NamedXContentRegistry getXContentRegistry() {
+        return delegate().getXContentRegistry();
+    }
+
+    @Override
+    public boolean isClosed() {
+        return delegate().isClosed();
+    }
+
+    @Override
+    public RestApiVersion getRestApiVersion() {
+        return delegate().getRestApiVersion();
+    }
+
+    @Override
+    public DeprecationHandler getDeprecationHandler() {
+        return delegate().getDeprecationHandler();
+    }
+
+    @Override
+    public void close() throws IOException {
+        delegate().close();
+    }
+}