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
this.newFieldsSeen = newFieldsSeen;
        this.dynamicObjectMappers = dynamicObjectMappers;
        this.dynamicRuntimeFields = dynamicRuntimeFields;
        this.id = id;
        this.version = version;
        this.seqID = seqID;
        this.dimensions = dimensions;
        this.parent = parent;
        this.dynamic = dynamic;
    }
    private DocumentParserContext(ObjectMapper parent, ObjectMapper.Dynamic dynamic, DocumentParserContext in) {
        this(
            in.mappingLookup,
            in.mappingParserContext,
            in.sourceToParse,
            in.ignoredFields,
            in.dynamicMappers,
            in.newFieldsSeen,
            in.dynamicObjectMappers,
            in.dynamicRuntimeFields,
            in.id,
            in.version,
            in.seqID,
            in.dimensions,
            parent,
            dynamic
        );
    }
    protected DocumentParserContext(
        MappingLookup mappingLookup,