Draft: Add support for ordering group runners by most active

What does this MR do and why?

• Changelog: added

This MR adds support for checking for the most active group runners. It does so by changing ee/app/finders/ee/ci/runners_finder.rb and the Ci::Runner model to allow sorting by most_active_desc for group runners when a group is specified.

Part of #452185 (closed), #437849 (closed)

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

Scenario	Screenshot
Using DIRECT membership
Not specifying membership
Using DESCENDANTS membership

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Create a group runner with the following tags: shell, gdk, mac, for instance on http://gdk.test:3000/groups/gitlab-org/-/runners

2. Create a simple pipeline on a project inside that group:

   default:
     tags:
       - shell
       - gdk
       - mac
   
   before_script:
     - echo "Before script section"
     - echo "For example you might run an update here or install a build dependency"
     - echo "Or perhaps you might print out some debugging details"
   
   after_script:
     - echo "After script section"
     - echo "For example you might do some cleanup here"
   
   build1:
     stage: build
     script:
       - echo "Do your build here"
       - sleep 30
   
   test1:
     stage: test
     script:
       - echo "Do a test here"
       - echo "For example run a test suite"
       - sleep 30
   
   test2:
     stage: test
     script:
       - echo "Do another parallel test here"
       - echo "For example run a lint test"
       - sleep 60
   
   deploy1:
     stage: deploy
     script:
       - echo "Do your deploy here"
     environment: production

3. Go to http://gdk.test:3000/-/graphql-explorer

4. Run the following query:

   {
     group(fullPath: "gitlab-org") {
       runners(first: 5, type: GROUP_TYPE, membership: DIRECT, sort: MOST_ACTIVE_DESC) {
         nodes {
           id
           jobs(statuses: RUNNING) {
             count
           }
         }
       }
     }
   }

Database query plans

Query most active group runners on group ID 9970 (gitlab-org)

SELECT "ci_runners".*
FROM "ci_runners"
  INNER JOIN "ci_runner_namespaces" ON "ci_runner_namespaces"."runner_id" = "ci_runners"."id"
  INNER JOIN (
    SELECT "ci_running_builds"."runner_id", ROW_NUMBER() OVER (PARTITION BY runner_id ORDER BY runner_id) AS rn
    FROM "ci_running_builds"
    WHERE "ci_running_builds"."runner_type" = 2
      AND "ci_running_builds"."runner_owner_namespace_xid" = 9970) AS "limited_builds" ON "limited_builds"."runner_id" = "ci_runners"."id"
  AND "limited_builds".rn <= 1000
WHERE "ci_runner_namespaces"."namespace_id" = 9970
  AND "ci_runners"."runner_type" = 2
GROUP BY "ci_runners"."id"
ORDER BY COUNT(limited_builds.runner_id) DESC NULLS LAST, "ci_runners"."id" DESC
LIMIT 5 OFFSET 0