release-23.1: schemachanger: address index validation errors during pk swap

[fqazi]

Backport 1/1 commits from #118843.

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Previously, when mutations were made public for validation queries, the declarative schema changer primary key swaps were not appropriately handled. This could lead to scenarios where queries would be unable to find columns in recreated secondary indexes that should ideally exist (i.e. columns from the old primary key). This would cause validation to fail on these newly created indexes. To address, this patch adds logic for making declarative primary key swaps mutations properly by detecting them.

Fixes: #118626

Release note (bug fix): ALTER PRIMARY KEY could fail with a "non-nullable column with no value! Index scanned .." when validating recreated secondary indexes.

Release justification: Low risk changes that fixes a regression that prevents us from being able ALTER PRIMARY KEY if any index exists accessing the old PK columns via an expression

[blathers-crl]

Thanks for opening a backport.

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[cockroach-teamcity]

This change is 

[rafiss]

lgtm! can you post in #db-backports-point-releases ?