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
{
              "metrics": {
                "service": {
                  "time" : 10,
                  "time.max" : 500
                }
              }
            }
            """));
        assertNull(doc.rootDoc().getField("time"));
        assertNull(doc.rootDoc().getField("time.max"));
        assertNull(doc.dynamicMappingsUpdate());
    }
    public void testWriteToFieldAlias() throws Exception {
        DocumentMapper mapper = createDocumentMapper(mapping(b -> {
            b.startObject("alias-field");
            {
                b.field("type", "alias");
                b.field("path", "concrete-field");
            }
            b.endObject();
            b.startObject("concrete-field").field("type", "keyword").endObject();
        }));
        DocumentParsingException exception = expectThrows(
            DocumentParsingException.class,
            () -> mapper.parse(source(b -> b.field("alias-field", "value")))
        );
        assertEquals("[1:16] Cannot write to a field alias [alias-field].", exception.getMessage());
    }
    public void testCopyToFieldAlias() throws Exception {