Add externalStatusChecks to graphql endpoint

What does this MR do and why?

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

Adds externalStatusChecks collection under project.branchRules to display information about the approval rule for a given branch

How to set up and validate locally

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

      # If you don't have any ExternalStatusChecks configured go to gdk.test:3000/flightjs/Flight/-/settings/merge_requests
      # and configure one in the `Status Checks` section
      status_check = MergeRequests::ExternalStatusCheck.last
      project = status_check.project
      # If you don't have any protected branches this will fail to link the approval rule to the
      # protected branch and therefore will not be returned in GraphiQL.
      # Go to gdk.test:3000/flightjs/Flight/-/settings/repository and create a protected branch and then link it 
      # with the code below.
      status_check.protected_branches << project.protected_branches.first if status_check.protected_branches.empty?
      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: "flightjs/Flight") {
        branchRules {
          nodes {
            externalStatusChecks {
              id
              name
              externalUrl
            }
          }
        }
      }
    }

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: "flightjs/Flight") {
        branchRules {
          nodes {
            externalStatusChecks {
              id
              name
              externalUrl
            }
          }
        }
      }
    }

MR acceptance checklist

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