Add deploy keys support for Protected tags

What does this MR do and why?

Contributes to #325415 (closed)

• Enable Deploy keys section on UI

Roadmap

Step	MR
Database migration to add deploy_key_id	!109795 (merged)
Add associations and validations	!109911 (merged)
Enable verification for protected tags creation	!110238 (merged)
Allow users to choose Deploy keys for protected tags	👈 this MR
Documentation	!110240 (merged)

Screenshots/Recordings

Testing

1. Enable the protected_refs_for_users feature flag
2. Navigate to project > Settings > Repository > Deploy tokens
3. Create a Deploy token
4. Navigate to project > Settings > Repository > Protected tags
5. Select a tag
6. The deploy token created in step 2 should be visible in the "Allowed to create" dropdown
7. Select the deploy token created in step 2 and click "Protect"
8. The Protected tag should be visible with the deploy token.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.