[CE] Backport SAML settings changes (Adds validators and rack cookie helper)
What does this MR do?
Backports changes from https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/4549
Why was this MR needed?
Keep CE and EE in sync
Does this MR meet the acceptance criteria?
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Changelog entry added, if necessary -
Tests added for this feature/bug - Review
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Has been reviewed by Backend
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End-to-end tests pass ( package-and-qa
manual pipeline job)