Add spannerio vector writer. (#36654)

* draft

* Simplify

* Add xlang test markers.

* Fix tests.

* lints

* Remove extractor_fn args.

* Linter.

---------

Co-authored-by: Claude <[email protected]>

Author: claudevdm

.github/trigger_files/beam_PostCommit_Python_Xlang_Gcp_Direct.json

@@ -1,4 +1,4 @@
 {
   "comment": "Modify this file in a trivial way to cause this test suite to run",
-  "modification": 12
+  "modification": 13
 }