Expose user and group for branch protection access levels in EE

What does this MR do and why?

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

This change is for EE only

Expose user and group nodes under project.branchRules[].branchProtection.{mergeAccessLevels,pushAccessLevels}.

The user object is a UserType and the group object is a GroupType. Both fields can be null. The fields will never both be present. An access rule can be either a user, group, or role rule.

How to set up and validate locally

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

      user_project = ProtectedBranch::PushAccessLevel.for_user.last.protected_branch.project
      group_project = ProtectedBranch::PushAccessLevel.for_group.last.protected_branch.project
      user_project.full_path
      group_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")
     user_project.add_guest(user)
     group_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 {
              pushAccessLevels {
                nodes {
                  user {
                    name
                  }
                  group {
                    name
                  }
                }
              }
            }
          }
        }
      }
    }

5. Make yourself a maintainer

     user_project.add_guest(user)
     group_project.add_guest(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 {
              pushAccessLevels {
                nodes {
                  user {
                    name
                  }
                  group {
                    name
                  }
                }
              }
            }
          }
        }
      }
    }

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