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
MappingParserContext mappingParserContext,
        SourceToParse source,
        ObjectMapper parent,
        ObjectMapper.Dynamic dynamic
    ) {
        this(
            mappingLookup,
            mappingParserContext,
            source,
            new HashSet<>(),
            new ArrayList<>(),
            new HashSet<>(),
            new HashMap<>(),
            new ArrayList<>(),
            null,
            null,
            null,
            DocumentDimensions.fromIndexSettings(mappingParserContext.getIndexSettings()),
            parent,
            dynamic
        );
    }
    public final IndexSettings indexSettings() {
        return mappingParserContext.getIndexSettings();
    }
    public final IndexAnalyzers indexAnalyzers() {
        return mappingParserContext.getIndexAnalyzers();
    }
    public final RootObjectMapper root() {
        return this.mappingLookup.getMapping().getRoot();