[RUN AS-IF-FOSS] Add association between DeployKey and PushAccessLevel

What does this MR do?

After that MR, this MR here is another round of extracting code from a bigger MR to introduce a new feature more gently (Deploy Keys selectable on Protected Branches)

This MR here focuses on establishing the new association, at the model level, between PushAccessLevel and DeployKey.

Screenshots (strongly suggested)

Some quick manual testing to make sure that we still maintain the current ("deploy-key-free") UI.

FOSS:

EE (no deploy key entries after users, in the dropdown):

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

Related to #30769 (closed)