sql: disallow other schema changes during an in progress primary key change

[jseldess]

PR: cockroachdb/cockroach#45397

From release notes:

The use of schema changes when a primary key change is in progress is now disabled. This includes the following: (1) running a primary key change in a transaction and then starting another schema change in the same transaction, and (2) starting a primary key change on one connection and then starting a schema change on the same table on another connection while the initial primary key change is currently executing. [#45397][#45397] {% comment %}doc{% endcomment %}

[awoods187]

alter pk https://airtable.com/tblD3oZPLJgGhCmch/viwNg2fK3NU3DUY6Z/reckXe8OwbdVi8WzH