GraphQL: Add CiRunnerMachine type

What does this MR do and why?

Describe in detail what your merge request does and why.

NOTE: This MR is probably best reviewed commit by commit, as I've tried to isolate each concern into its own commit.

This MR adds:

• a CiRunnerMachine type to represent the ci_runner_machines table;
• the policy for Ci::RunnerMachine;
• a machines field in the CiRunner type.

It also makes a minor change in the computation of the stale state for Ci::Runner for consistency with Ci::RunnerMachine, meaning that if the runner was not seen it 3 months or more, then it is considered stale. Until now, if the runner had been seen exactly 3 months ago, it would still not be considered stale. This has no practical implications in the software, but better be coherent across similar models.

Closes #395013 (closed)

Screenshots or screen recordings

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How to set up and validate locally

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

1. We'll first need to enable a couple of FFs in rails console:

   Feature.enable(:create_runner_workflow_for_admin)
   Feature.enable(:create_runner_machine)

2. We'll then need to create a new runner (you'll need the value from the ephemeralAuthenticationToken field in the following step):

   Open http://gdk.test:3000/-/graphql-explorer and enter the following mutation:

   mutation runnerCreate($input: RunnerCreateInput!) {
     runnerCreate(input: $input) {
       runner {
         id
         ephemeralAuthenticationToken
         contactedAt
       }
       errors
     }
   }

   with the following variables:

   {
     "input": {
       "description": "my awesome description",
       "maintenanceNote": "often.",
       "paused": true
     }
   }

3. We can now "register" a runner machine under the runner we just created:

   curl --request POST "http://gdk.test:3000/api/v4/runners/verify" --form "token=$EPHEMERAL_RUNNER_TOKEN" --form "system_id=s_some_system_id"

4. Finally, we can test this MR by listing the runner machines under the runner we created:

   {
     runners(sort: CREATED_ASC, last: 1) {
       nodes {
         id
         tagList
         machines {
           nodes {
             id
             createdAt
             contactedAt
             version
             revision
             status
             platformName
             architectureName
             executorName
             ipAddress
           }
         }
       }
     }
   }

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.