Allow url map to handle backend service fields that can also be backend buckets. (#4269) (#2754)

* undo the removal of the resourcerefs, write a function specifically for backend refs.

* Add test with bucket, add comment.

Signed-off-by: Modular Magician <[email protected]>

Author: modular-magician

.changelog/4269.txt

@@ -0,0 +1,3 @@
+```release-note:enhancement
+compute: `google_compute_url_map`'s fields referring to backend services can now also refer to backend buckets.
+```