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/test/java/org/elasticsearch/index/mapper/DocumentParserTests.java
DocumentParsingException e = expectThrows(
            DocumentParsingException.class,
            () -> mapper.parse(source(b -> b.field("_field_names", 0)))
        );
        assertTrue(
            e.getCause().getMessage(),
            e.getCause().getMessage().contains("Field [_field_names] is a metadata field and cannot be added inside a document.")
        );
        mapper.parse(source(b -> b.field("foo._field_names", 0))); 
    }
    public void testDocumentContainsAllowedMetadataField() throws Exception {
        DocumentMapper mapper = createDocumentMapper("""
            { "_doc": { "dynamic" : "strict", "properties" : { "object" : { "type" : "object" } } } }
            """);
        {
            
            DocumentParsingException e = expectThrows(
                DocumentParsingException.class,
                () -> mapper.parse(source(b -> b.nullField(DocumentParserTestsPlugin.MockMetadataMapper.CONTENT_TYPE)))
            );
            assertTrue(e.getMessage(), e.getMessage().contains("failed to parse field [_mock_metadata]"));
        }
        {
            
            DocumentParsingException e = expectThrows(
                DocumentParsingException.class,
                () -> mapper.parse(
                    source(
                        b -> b.field(DocumentParserTestsPlugin.MockMetadataMapper.CONTENT_TYPE)
                            .startObject()