docs: samples and tests for admin backup APIs

[rahul2393]

Notes

• Adds samples and tests for auto-generated backup admin APIs.
• The sample code style is mostly consistent with our existing samples.

[snippet-bot]

Here is the summary of changes.

You are about to add 12 region tags.

• samples/samples/admin/backup_snippet.py:28, tag spanner_create_backup
• samples/samples/admin/backup_snippet.py:69, tag spanner_create_backup_with_encryption_key
• samples/samples/admin/backup_snippet.py:117, tag spanner_restore_backup
• samples/samples/admin/backup_snippet.py:151, tag spanner_restore_backup_with_encryption_key
• samples/samples/admin/backup_snippet.py:196, tag spanner_cancel_backup_create
• samples/samples/admin/backup_snippet.py:246, tag spanner_list_backup_operations
• samples/samples/admin/backup_snippet.py:298, tag spanner_list_database_operations
• samples/samples/admin/backup_snippet.py:328, tag spanner_list_backups
• samples/samples/admin/backup_snippet.py:416, tag spanner_delete_backup
• samples/samples/admin/backup_snippet.py:455, tag spanner_update_backup
• samples/samples/admin/backup_snippet.py:488, tag spanner_create_database_with_version_retention_period
• samples/samples/admin/backup_snippet.py:539, tag spanner_copy_backup

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