Introduce a new gitlab:doctor:encryption_keys task

What does this MR do and why?

Backport part of Introduce Gitlab::Encryption::KeyProvider for e... (!186541 - merged) • Rémy Coutable • 17.11 and Introduce a new gitlab:doctor:encryption_keys task (!187500 - merged) • Rémy Coutable • 17.11 entirely to %17.9.

I've set bugavailability since the issue this will help to diagnose is potentially breaking the availability of some endpoints: Active Record Decryption Error since upgrading ... (#527923 - closed) • Rémy Coutable • 18.0

References

• Closes Backport Gitlab::Encryption::KeyProvider to 17.9 (#535242 - closed)
• Related to Active Record Decryption Error since upgrading ... (#527923 - closed)

MR acceptance checklist

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• This MR is backporting a bug fix, documentation update, or spec fix, previously merged in the default branch.
• The MR that fixed the bug on the default branch has been deployed to GitLab.com (not applicable for documentation or spec changes).
• This MR has a severity label assigned (if applicable).
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• Ensure the e2e:test-on-omnibus-ee job has either succeeded or been approved by a Software Engineer in Test.

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