changefeed: support primary key columns which are not visible

[fqazi]

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Previously, when schema changes were happening with an active change feed, it was possible for ALTER PRIMARY KEY USING HASH to have an intermediate state where the hash column could exist within the primary key without being public. This is expected since complex schema changes could not roll back if these columns were made completely public at the same time as the primary key. This would break assumptions within CDC, which expected all primary key columns to be public. To address this, this patch modifies this logic to ignore columns that are writable but not yet fully readable in the primary key.

Fixes: #128457

Release note: None

[cockroach-teamcity]

This change is 

[fqazi]

@asg0451 TFTR!

bors r+

[craig[bot]]

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