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
context.version(),
            context.seqID(),
            context.id(),
            source.routing(),
            context.reorderParentAndGetDocs(),
            context.sourceToParse().source(),
            context.sourceToParse().getXContentType(),
            dynamicUpdate
        ) {
            @Override
            public String documentDescription() {
                IdFieldMapper idMapper = (IdFieldMapper) mappingLookup.getMapping().getMetadataMapperByName(IdFieldMapper.NAME);
                return idMapper.documentDescription(this);
            }
        };
    }
    private static void internalParseDocument(MetadataFieldMapper[] metadataFieldsMappers, DocumentParserContext context) {
        try {
            final boolean emptyDoc = isEmptyDoc(context.root(), context.parser());
            for (MetadataFieldMapper metadataMapper : metadataFieldsMappers) {
                metadataMapper.preParse(context);
            }
            if (context.root().isEnabled() == false) {
                
                context.parser().skipChildren();
            } else if (emptyDoc == false) {
                parseObjectOrNested(context);
            }
            executeIndexTimeScripts(context);
            for (MetadataFieldMapper metadataMapper : metadataFieldsMappers) {