feat(spanner): Add samples for backup schedule feature APIs

[aman-19]

This PR adds code samples for the following Cloud Spanner APIs:

1. CreateBackupSchedule (both full and incremental backup schedules)
2. ListBackupSchedules
3. GetBackupSchedule
4. UpdateBackupSchedule
5. DeleteBackupSchedule

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This change is 

[snippet-bot]

Here is the summary of changes.

You are about to add 6 region tags.

• google/cloud/spanner/samples/samples.cc:1631, tag spanner_create_full_backup_schedule
• google/cloud/spanner/samples/samples.cc:1677, tag spanner_create_incremental_backup_schedule
• google/cloud/spanner/samples/samples.cc:1723, tag spanner_delete_backup_schedule
• google/cloud/spanner/samples/samples.cc:1752, tag spanner_update_backup_schedule
• google/cloud/spanner/samples/samples.cc:1804, tag spanner_get_backup_schedule
• google/cloud/spanner/samples/samples.cc:1834, tag spanner_list_backup_schedules

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[scotthart]

/gcbrun

[codecov]

Codecov Report

Attention: Patch coverage is 13.74046% with 113 lines in your changes missing coverage. Please review.

Project coverage is 93.45%. Comparing base (f197a85) to head (2b00801).
Report is 5 commits behind head on main.

Files with missing lines	Patch %	Lines
google/cloud/spanner/samples/samples.cc	13.74%	113 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #14720      +/-   ##
==========================================
- Coverage   93.50%   93.45%   -0.06%     
==========================================
  Files        2323     2323              
  Lines      207992   208123     +131     
==========================================
+ Hits       194478   194494      +16     
- Misses      13514    13629     +115     

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[dbolduc]

/gcbrun

[dbolduc]

Thanks! Feel free to "Squash and merge"

I think you have permissions to do this, but if not, let me know, and I can merge the PR.

[aman-19]

It seems I don't have the permission. Could you please help merging this? Thanks.