Expose id field for protected ref access entities in API

For #20229 (closed) we want to introduce the ability to update access levels. We plan to follow the convention that is introduced for Protected Environments API. It follows the same conventions as Rails Nested Attributes.

As a prerequisite, we need to expose the id attribute in the ProtectedRefAccess entity and update the documentation for

• Protected Branches API: https://docs.gitlab.com/ee/api/protected_branches.html
• Protected Tags API: https://docs.gitlab.com/ee/api/protected_tags.html