chore(samples): Add samples for Cloud Spanner Default Backup Schedules

[Sally-Ye]

This PR adds code samples for cloud spanner default backup schedules using the following apis:

1. CreateInstance
2. UpdateInstance

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Fixes #3448 ☕️

If you write sample code, please follow the samples format.

[snippet-bot]

Here is the summary of changes.

You are about to add 2 region tags.

• samples/snippets/src/main/java/com/example/spanner/CreateInstanceWithoutDefaultBackupSchedulesExample.java:19, tag spanner_create_instance_without_default_backup_schedule
• samples/snippets/src/main/java/com/example/spanner/UpdateInstanceDefaultBackupScheduleTypeExample.java:19, tag spanner_update_instance_default_backup_schedule_type

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