feat: Create a few code snippets as examples for using Spanner Graph in Python (#1186)

* Create a set of code snippets for using Graph on Cloud Spanner

* Update to match gcloud/cli examples that exist in the docs

* Fix update with graph query predicate syntax

* Added an update step for allowing commit timestamps and changed to schema to not have that option

* Fix styling using flake8

* Add tests for new Spanner Graph snippets

* Fix some region tags that were inconsistent

* Remove one unnecessary function and some redundant comments

* Remove reference to allow_commit_timestamp

* Fix lint issues in test file

---------

Co-authored-by: Sri Harsha CH <[email protected]>

Author: bharadwajvr