Currently, when an object is mapped with `subobjects: false`, trying to ingest a document that has nested objects leads to a mapping exception.

In a couple of occasions, we've been talking about that it would be convenient if Elasticsearch could automatically flatten the fields instead of the client having to know about the `subobjects` setting. This would make `subobjects: false` more of a drop-in replacement that allows for a much easier adoption, without changes in the client.

Note that the `_source` should ideally stay untouched, just the stored fields should be flattened automatically.

Some related issues
* https://github.com/elastic/apm/issues/347
* https://github.com/elastic/elasticsearch/issues/88777
* https://github.com/elastic/package-spec/issues/349
server/src/test/java/org/elasticsearch/index/mapper/DocumentParserTests.java
private static final String CONTENT_TYPE = "_mock_metadata";
            private static final String FIELD_NAME = "_mock_metadata";
            protected MockMetadataMapper() {
                super(new KeywordFieldMapper.KeywordFieldType(FIELD_NAME));
            }
            @Override
            protected void parseCreateField(DocumentParserContext context) throws IOException {
                if (context.parser().currentToken() == XContentParser.Token.VALUE_STRING) {
                    context.doc().add(new StringField(FIELD_NAME, context.parser().text(), Field.Store.YES));
                } else {
                    throw new IllegalArgumentException("Field [" + fieldType().name() + "] must be a string.");
                }
            }
            @Override
            protected String contentType() {
                return CONTENT_TYPE;
            }
            @Override
            public SourceLoader.SyntheticFieldLoader syntheticFieldLoader() {
                return new StringStoredFieldFieldLoader(name(), simpleName(), null) {
                    @Override
                    protected void write(XContentBuilder b, Object value) throws IOException {
                        BytesRef ref = (BytesRef) value;
                        b.utf8Value(ref.bytes, ref.offset, ref.length);
                    }
                };
            }
            private static final TypeParser PARSER = new FixedTypeParser(c -> new MockMetadataMapper());
        }
        @Override