Use Class.getDeclaredField() instead of getField() when fetching …

…Thrift metadata. (#1729)

Related: #1728

Motivation:

When a user builds a `.thrift` file with the `private-members` option enabled,
Armeria fails to find some fields in the generated code.

Modifications:

- Use `Class.getDeclaredField()` instead of `getField()`.

Result:

- Fixes #1728

thrift/src/main/java/com/linecorp/armeria/internal/thrift/ThriftFunction.java

@@ -102,7 +102,7 @@ private ThriftFunction(
                     continue;
                 }
 
-                final Class<?> fieldType = resultType.getField(fieldName).getType();
+                final Class<?> fieldType = resultType.getDeclaredField(fieldName).getType();
                 if (Throwable.class.isAssignableFrom(fieldType)) {
                     @SuppressWarnings("unchecked")
                     final Class<Throwable> exceptionFieldType = (Class<Throwable>) fieldType;