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 <57220027+harshachinta@users.noreply.github.com>

Author: bharadwajvr