Add deploy keys to protected branches (DB and API)

What does this MR do?

This first MR to allow deploy keys to push onto protected branches (related issue)

It does two things:

• it adds a deploy_key_id column to the protected_branch_push_access_levels table.
• it adds a new API endpoint: GET /autocomplete/deploy_keys_with_owners which payload returns enabled deploy keys with details about their owner for each of them, for a specific project.

This is what's necessary to then get things going on frontend development. Another backend MR will be needed to persist associations between specific protected branches and deploy keys.

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Relates to #30769 (closed)