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
NoOpObjectMapper(String name, String fullPath) {
            super(name, fullPath, Explicit.IMPLICIT_TRUE, Explicit.IMPLICIT_TRUE, Dynamic.RUNTIME, Collections.emptyMap());
        }
    }
    /**
     * Internal version of {@link DocumentParserContext} that is aware of implementation details like nested documents
     * and how they are stored in the lucene index.
     */
    private static class RootDocumentParserContext extends DocumentParserContext {
        private final ContentPath path = new ContentPath();
        private final XContentParser parser;
        private final LuceneDocument document;
        private final List<LuceneDocument> documents = new ArrayList<>();
        private final long maxAllowedNumNestedDocs;
        private long numNestedDocs;
        private boolean docsReversed = false;
        RootDocumentParserContext(
            MappingLookup mappingLookup,
            MappingParserContext mappingParserContext,
            SourceToParse source,
            XContentParser parser