schemachanger: add `checkNoRegionChangeUnderway` checks

[annrpom]

In TestRegionChangeRacingRegionalByRowChange, we ensure that firing off an ADD/DROP REGION whilst an index-related change on an RBR table is happening (and vice versa) fails properly.

In the declarative schema changer, this means we need to ensure that no regions on the database are being modified before we can proceed with the schema change -- like in drop index https://github.com/cockroachdb/cockroach/blob/9f8f1e8ed68700ece9f8d343b515654de81ff801/pkg/sql/drop_index.go#L330-L338

In other words, this tracks the implementation of checkNoRegionChangeUnderway in the DSC.

Jira issue: CRDB-44972

[annrpom]

This check is needed for the following schema changes on an RBR table:

• DROP INDEX
• ADD COLUMN
• ALTER PRIMARY KEY
• ADD UNIQUE CONSTRAINT
• CREATE INDEX