docs: samples and tests for admin database APIs

[rahul2393]

Notes

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

depends on: #1065

[snippet-bot]

Here is the summary of changes.

You are about to add 28 region tags.

• samples/generated/backup_sample.py:27, tag spanner_create_backup
• samples/generated/backup_sample.py:65, tag spanner_create_backup_with_encryption_key
• samples/generated/backup_sample.py:109, tag spanner_restore_backup
• samples/generated/backup_sample.py:140, tag spanner_restore_backup_with_encryption_key
• samples/generated/backup_sample.py:180, tag spanner_cancel_backup_create
• samples/generated/backup_sample.py:212, tag spanner_list_backup_operations
• samples/generated/backup_sample.py:252, tag spanner_list_database_operations
• samples/generated/backup_sample.py:274, tag spanner_list_backups
• samples/generated/backup_sample.py:338, tag spanner_delete_backup
• samples/generated/backup_sample.py:361, tag spanner_update_backup
• samples/generated/backup_sample.py:383, tag spanner_create_database_with_version_retention_period
• samples/generated/backup_sample.py:427, tag spanner_copy_backup
• samples/generated/pg_snippets.py:32, tag spanner_postgresql_create_table_with_datatypes
• samples/generated/pg_snippets.py:67, tag spanner_postgresql_jsonb_add_column
• samples/generated/pg_snippets.py:113, tag spanner_postgresql_insert_datatypes_data
• samples/generated/snippets.py:31, tag spanner_create_instance
• samples/generated/snippets.py:64, tag spanner_create_database_with_default_leader
• samples/generated/snippets.py:107, tag spanner_add_and_drop_database_role
• samples/generated/snippets.py:150, tag spanner_create_table_with_datatypes
• samples/generated/snippets.py:187, tag spanner_add_json_column
• samples/generated/snippets.py:217, tag spanner_add_numeric_column
• samples/generated/snippets.py:244, tag spanner_create_table_with_foreign_key_delete_cascade
• samples/generated/snippets.py:287, tag spanner_alter_table_with_foreign_key_delete_cascade
• samples/generated/snippets.py:321, tag spanner_drop_foreign_key_constraint_delete_cascade
• samples/generated/snippets.py:352, tag spanner_create_sequence
• samples/generated/snippets.py:403, tag spanner_alter_sequence
• samples/generated/snippets.py:450, tag spanner_drop_sequence
• samples/generated/snippets.py:480, tag spanner_insert_data

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