docs: add scheduled query samples

[tswast]

Add Python samples for all in the scheduled query documentation:
https://cloud.google.com/bigquery/docs/scheduling-queries

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Towards internal issue 175211322 (list configs sample)
Towards internal issue 143380740 (scheduled queries samples)

[snippet-bot]

Here is the summary of changes.

You added 14 region tags.

• samples/snippets/manage_transfer_configs.py:17, tag bigquerydatatransfer_list_configs
• samples/snippets/manage_transfer_configs.py:27, tag bigquerydatatransfer_list_configs
• samples/snippets/manage_transfer_configs.py:38, tag bigquerydatatransfer_update_config
• samples/snippets/manage_transfer_configs.py:53, tag bigquerydatatransfer_update_config
• samples/snippets/manage_transfer_configs.py:73, tag bigquerydatatransfer_update_credentials
• samples/snippets/manage_transfer_configs.py:90, tag bigquerydatatransfer_update_credentials
• samples/snippets/manage_transfer_configs.py:109, tag bigquerydatatransfer_schedule_backfill
• samples/snippets/manage_transfer_configs.py:123, tag bigquerydatatransfer_schedule_backfill
• samples/snippets/manage_transfer_configs.py:151, tag bigquerydatatransfer_delete_transfer
• samples/snippets/manage_transfer_configs.py:164, tag bigquerydatatransfer_delete_transfer
• samples/snippets/scheduled_query.py:17, tag bigquerydatatransfer_create_scheduled_query
• samples/snippets/scheduled_query.py:18, tag bigquerydatatransfer_create_scheduled_query_with_service_account
• samples/snippets/scheduled_query.py:41, tag bigquerydatatransfer_create_scheduled_query
• samples/snippets/scheduled_query.py:42, tag bigquerydatatransfer_create_scheduled_query_with_service_account

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