Add mergeApprovalLevels to GraphQL endpoint

What does this MR do and why?

#362706 (closed) -> #372362 (closed)

Adds mergeApprovalLevels node under project.branchRules[].branchProtection to display information about the merge access protections for a given branch

Currently only supports CE mode. A later MR will add additional values for EE installations.

How to set up and validate locally

1. Find a project.full_path of a project that has at least one protected branch

      project = ProtectedBranch::MergeAccessLevel.last.protected_branch.project
      project.full_path

2. Test permissions by assigning yourself as a guest to the project

     # user = User.find_by(email: "YOUR_EMAIL") # Use this if you do not use the default admin user in GDK
     user = User.find_by(email: "admin@example.com")
     project.add_guest(user)

3. Visit http://gdk.test:3000/-/graphql-explorer
4. Execute the following query, replace the full path value (as guest you should not see any rules)

    {
      project(fullPath: "FULL_PATH") {
        branchRules {
          nodes {
            branchProtection {
              mergeAccessLevels {
                nodes {
                  accessLevel
                  accessLevelDescription
                }
              }
            }
          }
        }
      }
    }

5. Make yourself a maintainer

     project.add_maintainer(user)

6. Execute the following query, replace the full path value (as maintainer you should see all the rules)

    {
      project(fullPath: "FULL_PATH") {
        branchRules {
          nodes {
            branchProtection {
              mergeAccessLevels {
                nodes {
                  accessLevel
                  accessLevelDescription
                }
              }
            }
          }
        }
      }
    }

MR acceptance checklist

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