GraphQL: Add runnerMachine field to CiJob type

What does this MR do and why?

Describe in detail what your merge request does and why.

This MR is best reviewed commit-by-commit, as it touches 3 related areas:

• Adds a systemId field to CiRunnerMachine;
• Adds a Preloaders::RunnerMachinePolicyPreloader to do the necessary preloading for the Ci::RunnerPolicy class;
• Adds a runnerMachine field to CiJob (which relies on the preloader to avoid N+1 queries).

There is no changelog trailer since the new fields are marked as alpha.

Closes #395783 (closed)

Screenshots or screen recordings

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

How to set up and validate locally

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

1. Go to the shell in your GDK gitlab directory and run bundle exec rake "gitlab:seed:runner_fleet". This will seed your GDK with some runners and jobs required for testing this MR.

2. Open the GDK console (rails console in a shell), and run the following commands, taking note of the GraphQL GID at the end:

   > runner = Ci::Runner.instance_type.first
   > runner.builds.first.update!(runner_machine: Ci::RunnerMachine.new(runner: runner, system_xid: "s_test_id1"))
   > runner.builds.second.update!(runner_machine: Ci::RunnerMachine.new(runner: runner, system_xid: "s_test_id2"))
   > runner.to_global_id.to_s
   => "gid://gitlab/Ci::Runner/1"

3. Open http://gdk.test:3000/-/graphql-explorer

   {
     runner(id: "gid://gitlab/Ci::Runner/1") {
       jobs(last: 5) {
         nodes {
           id
           name
           runnerMachine {
             architectureName
             contactedAt
             createdAt
             executorName
             id
             ipAddress
             platformName
             revision
             status
             systemId
             version
           }
         }
       }
     }
   }

The runner should list the 2 runner machines that were just created (one in each of the 2 jobs, the remaining being null values).

The application log should show that the 5 runners and 2 runner machines are fetched in one go, instead of one request per record:

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.