schemadiff: validate columns referenced by FOREIGN KEY

[shlomi-noach]

Description

This PR validates that columns covered by a foreign key do in fact exist. Note that this validation only applies to current table, and does not validate the referenced columns in the parent table.

Related Issue(s)

#10203
#10189

Checklist

• "Backport me!" label has been added if this change should be backported
• Tests were added or are not required
• Documentation was added or is not required

Deployment Notes

[github-actions]

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[shlomi-noach]

Upgrade Downgrade Testing Query Serving / Run Upgrade Downgrade Test (pull_request) is too flaky. Gonna merge this PR even though it's red.