Seamlessly propagate schema changes made after pipelines starts running for Spanner Change Streams to BigQuery template

[ChangyuLi28]

This pr supports schema updates after pipeline starts running.

• Before you add a new column to an existing tracked Cloud Spanner table, first add the column to the BigQuery changelog table. The new column must be NULLABLE. Then add the column to the Cloud Spanner table. The new column is automatically populated when the pipeline receives a new record with that column. It's recommended to wait for a couple of minutes after making schema updates in BigQuery to start the load including the new column because of this issue.
• To add a new table, add the table in the Cloud Spanner database first. The table is automatically created when the pipeline receives a record for the new table.
• The template doesn't drop tables or columns from BigQuery. If a column is dropped from the Cloud Spanner table, then null values are populated to these changelog columns for records generated after the columns are dropped from the Cloud Spanner table, unless you manually drop the column from BigQuery.
• The template doesn't support column type updates or column nullable mode updates.

Follow this guide to deploy pipelines with this template. More specific instructions.

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[ChangyuLi28]

Integration tests will be added in a future PR. Do not merge now.