feat: Add samples for backup schedule feature APIs.

[aman-19]

This PR adds code samples for the following APIs:

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

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Fixes #3338 ☕️

[snippet-bot]

Here is the summary of changes.

You are about to add 6 region tags.

• samples/snippets/src/main/java/com/example/spanner/CreateFullBackupScheduleSample.java:19, tag spanner_create_full_backup_schedule
• samples/snippets/src/main/java/com/example/spanner/CreateIncrementalBackupScheduleSample.java:19, tag spanner_create_incremental_backup_schedule
• samples/snippets/src/main/java/com/example/spanner/DeleteBackupScheduleSample.java:19, tag spanner_delete_backup_schedule
• samples/snippets/src/main/java/com/example/spanner/GetBackupScheduleSample.java:19, tag spanner_get_backup_schedule
• samples/snippets/src/main/java/com/example/spanner/ListBackupSchedulesSample.java:19, tag spanner_list_backup_schedules
• samples/snippets/src/main/java/com/example/spanner/UpdateBackupScheduleSample.java:19, tag spanner_update_backup_schedule

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[ketanv3]

LGTM! Few minor changes needed.

[ketanv3]

Thanks for the changes, @aman-19. LGTM!