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
assertTrue(fooMapper instanceof ObjectMapper);
        Mapper barMapper = ((ObjectMapper) fooMapper).getMapper("bar");
        assertTrue(barMapper instanceof ObjectMapper);
        Mapper baz = ((ObjectMapper) barMapper).getMapper("baz");
        assertNotNull(baz);
        assertEquals("foo.bar.baz", baz.name());
        assertEquals("baz", baz.simpleName());
        List<IndexableField> fields = doc.rootDoc().getFields("foo.bar.baz");
        assertEquals(2, fields.size());
        String[] fieldStrings = fields.stream().map(Object::toString).toArray(String[]::new);
        assertArrayEquals(new String[] { "LongField <foo.bar.baz:1>", "LongField <foo.bar.baz:2>" }, fieldStrings);
        
        Mapping newMapping = MapperService.mergeMappings(mapperService.documentMapper(), mapping, MapperService.MergeReason.MAPPING_UPDATE);
        DocumentMapper newDocMapper = new DocumentMapper(mapperService.documentParser(), newMapping, newMapping.toCompressedXContent());
        ParsedDocument doc2 = newDocMapper.parse(source("""
            {
              "foo" : {
                "bar" : {
                  "baz" : 10
                }
              }
            }
            """));
        assertNull(doc2.dynamicMappingsUpdate());
        List<IndexableField> fields2 = doc2.rootDoc().getFields("foo.bar.baz");
        assertEquals(1, fields2.size());
        assertEquals("LongField <foo.bar.baz:10>", fields2.get(0).toString());
    }
    public void testDeeplyNestedDocument() throws Exception {
        int depth = 10000;