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
public Map<String, MetadataFieldMapper.TypeParser> getMetadataMappers() {
            return Collections.singletonMap(MockMetadataMapper.CONTENT_TYPE, MockMetadataMapper.PARSER);
        }
        @Override
        public Map<String, RuntimeField.Parser> getRuntimeFields() {
            return Collections.singletonMap("test-composite", new RuntimeField.Parser(n -> new RuntimeField.Builder(n) {
                @Override
                protected RuntimeField createRuntimeField(MappingParserContext parserContext) {
                    return new TestRuntimeField(
                        n,
                        List.of(
                            new TestRuntimeField.TestRuntimeFieldType(n + ".foo", KeywordFieldMapper.CONTENT_TYPE),
                            new TestRuntimeField.TestRuntimeFieldType(n + ".bar", KeywordFieldMapper.CONTENT_TYPE)
                        )
                    );
                }
                @Override
                protected RuntimeField createChildRuntimeField(
                    MappingParserContext parserContext,
                    String parentName,
                    Function<SearchLookup, CompositeFieldScript.LeafFactory> parentScriptFactory,
                    OnScriptError onScriptError
                ) {
                    throw new UnsupportedOperationException();
                }
            }));
        }
    }
}