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
.field("sub-field", "true")
                            .endObject()
                    )
                )
            );
            assertTrue(e.getMessage(), e.getMessage().contains("failed to parse field [_mock_metadata]"));
        }
        {
            ParsedDocument doc = mapper.parse(
                source(b -> b.field(DocumentParserTestsPlugin.MockMetadataMapper.CONTENT_TYPE, "mock-metadata-field-value"))
            );
            IndexableField field = doc.rootDoc().getField(DocumentParserTestsPlugin.MockMetadataMapper.CONTENT_TYPE);
            assertEquals("mock-metadata-field-value", field.stringValue());
        }
        {
            Exception e = expectThrows(DocumentParsingException.class, () -> mapper.parse(source(b -> {
                b.startObject("object");
                b.field(DocumentParserTestsPlugin.MockMetadataMapper.CONTENT_TYPE, "mock-metadata-field-value");
                b.endObject();
            })));
            assertTrue(e.getMessage(), e.getMessage().contains("dynamic introduction of [_mock_metadata] within [object]"));
        }
    }
    public void testSimpleMapper() throws Exception {
        DocumentMapper docMapper = createDocumentMapper(mapping(b -> {
            b.startObject("name");
            {
                b.field("type", "object");
                b.startObject("properties");
                {