Added DeployKeyAccess class

What does this MR do?

The goal of this MR is to extract some code from that other bigger MR.

The extracted code is related to introducing the DeployKeyAccess class. It'll be used to define whether a deploy key can access a protected branch.

This MR also updates the ProtectedBranch::PushAccessLevel class. These changes are related to content in DeployKeyAccess, and hidden behind the :deploy_keys_on_protected_branches feature flag.

Related issue: #30769 (closed)

Screenshots (strongly suggested)

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)