Add samples for Spanner Backup Schedule Apis

[AayushKadam]

Adds samples for the following APIs:

1. CreateBackupSchedule
2. GetBackupSchedule
3. ListBackupSchedules
4. UpdateBackupSchedule
5. DeleteBackupSchedule

[snippet-bot]

Here is the summary of changes.

You are about to add 5 region tags.

• spanner/spanner_create_backup_schedule_config.rb:15, tag spanner_create_backup_schedule_config
• spanner/spanner_delete_backup_schedule_config.rb:15, tag spanner_delete_backup_schedule_config
• spanner/spanner_get_backup_schedule_config.rb:15, tag spanner_get_backup_schedule_config
• spanner/spanner_list_backup_schedules_config.rb:15, tag spanner_list_backup_schedules_config
• spanner/spanner_update_backup_schedule_config.rb:15, tag spanner_update_backup_schedule_config

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