[BE] Allow editing of job token permissions

Why are we doing this work?

• Following from discussion, gitlab-org/gitlab#482107[Project____Project_-_Job_token_permissions_-_Restricting_allowlist.png] (comment 2153014920)
• Currently, we don't have any backend (GraphQL mutations, REST API endpoints or services) to edit a CI allowlist entry

Implementation Plan

• Create a service to allow job token permissions to be updated.
• Create GraphQL mutation to update only job token permissions (target cannot be updated)
• Create REST API to do the same^? (TBD)

Verification steps

• The user is able to edit permissions using GraphQL/REST API and they are reflected in the database