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/main/java/org/elasticsearch/index/mapper/DocumentParser.java
throw new UnsupportedOperationException();
            }
            @Override
            public String typeName() {
                throw new UnsupportedOperationException();
            }
            @Override
            public Query termQuery(Object value, SearchExecutionContext context) {
                throw new UnsupportedOperationException();
            }
        },
        FieldMapper.MultiFields.empty(),
        FieldMapper.CopyTo.empty()
    ) {
        @Override
        protected void parseCreateField(DocumentParserContext context) {
            
        }
        @Override
        public String name() {
            throw new UnsupportedOperationException();
        }
        @Override
        public String typeName() {
            throw new UnsupportedOperationException();
        }
        @Override
        public MappedFieldType fieldType() {
            throw new UnsupportedOperationException();
        }