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/DocumentParserContext.java
*
     * Use this if an exists query run against the field cannot use docvalues
     * or norms.
     */
    public final void addToFieldNames(String field) {
        FieldNamesFieldMapper fieldNamesFieldMapper = (FieldNamesFieldMapper) getMetadataMapper(FieldNamesFieldMapper.NAME);
        if (fieldNamesFieldMapper != null) {
            fieldNamesFieldMapper.addFieldNames(this, field);
        }
    }
    public final Field version() {
        return this.version;
    }
    public final void version(Field version) {
        this.version = version;
    }
    public final String id() {
        if (id == null) {
            assert false : "id field mapper has not set the id";
            throw new IllegalStateException("id field mapper has not set the id");
        }
        return id;
    }
    public final void id(String id) {
        this.id = id;
    }
    public final SeqNoFieldMapper.SequenceIDFields seqID() {
        return this.seqID;
    }
    public final void seqID(SeqNoFieldMapper.SequenceIDFields seqID) {