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
private final XContentParserConfiguration parserConfiguration;
    private final Supplier<DocumentParsingObserver> documentParsingObserverSupplier;
    private final MappingParserContext mappingParserContext;
    DocumentParser(
        XContentParserConfiguration parserConfiguration,
        MappingParserContext mappingParserContext,
        Supplier<DocumentParsingObserver> documentParsingObserverSupplier
    ) {
        this.mappingParserContext = mappingParserContext;
        this.parserConfiguration = parserConfiguration;
        this.documentParsingObserverSupplier = documentParsingObserverSupplier;
    }
    /**
     * Parse a document
     *
     * @param source        the document to parse
     * @param mappingLookup the mappings information needed to parse the document
     * @return the parsed document
     * @throws DocumentParsingException whenever there's a problem parsing the document
     */
    public ParsedDocument parseDocument(SourceToParse source, MappingLookup mappingLookup) throws DocumentParsingException {
        if (source.source() != null && source.source().length() == 0) {
            throw new DocumentParsingException(new XContentLocation(0, 0), "failed to parse, document is empty");
        }