Add a SpiService marker interface (#1427)

api/maven-api-spi/src/main/java/org/apache/maven/api/spi/ExtensibleEnumProvider.java

@@ -22,7 +22,7 @@
 
 import org.apache.maven.api.ExtensibleEnum;
 
-public interface ExtensibleEnumProvider<T extends ExtensibleEnum> {
+public interface ExtensibleEnumProvider<T extends ExtensibleEnum> extends SpiService {
 
     Collection<T> provides();
 }

api/maven-api-spi/src/main/java/org/apache/maven/api/spi/ModelParser.java

@@ -33,7 +33,7 @@
  * This allows plugging in additional syntaxes for the main model read by Maven when building a project.
  */
 @Experimental
-public interface ModelParser {
+public interface ModelParser extends SpiService {
 
     /**
      * Locates the pom in the given directory.

api/maven-api-spi/src/main/java/org/apache/maven/api/spi/PropertyContributor.java

@@ -27,7 +27,7 @@
  * very early, while there is no session created yet.
  */
 @Experimental
-public interface PropertyContributor {
+public interface PropertyContributor extends SpiService {
     /**
      * Invoked just before session is created with a mutable map that carries collected user properties so far.
      *

api/maven-api-spi/src/main/java/org/apache/maven/api/spi/SpiService.java

@@ -0,0 +1,29 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+package org.apache.maven.api.spi;
+
+import org.apache.maven.api.annotations.Consumer;
+import org.apache.maven.api.annotations.Experimental;
+
+/**
+ * Marker interface to indicate services that can be provided by plugins and extensions.
+ */
+@Experimental
+@Consumer
+public interface SpiService {}