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
@Override
        public MultiFields multiFields() {
            throw new UnsupportedOperationException();
        }
        @Override
        public Iterator<Mapper> iterator() {
            throw new UnsupportedOperationException();
        }
        @Override
        protected void doValidate(MappingLookup mappers) {
            throw new UnsupportedOperationException();
        }
        @Override
        protected void checkIncomingMergeType(FieldMapper mergeWith) {
            throw new UnsupportedOperationException();
        }
        @Override
        public Builder getMergeBuilder() {
            throw new UnsupportedOperationException();
        }
        @Override
        public XContentBuilder toXContent(XContentBuilder builder, Params params) {
            throw new UnsupportedOperationException();
        }
        @Override
        protected String contentType() {
            throw new UnsupportedOperationException();
        }
    };
    private static class NoOpObjectMapper extends ObjectMapper {