Add CreateRunner Mutation Endpoint

What does this MR do and why?

As part of building out the mutation to create runners, this MR introduces the mutation endpoint. The service to create the runner is introduced in !110076 (merged) and should be merged before this MR. The policies to authorize creating runners with this method are behind the feature flag create_runner_workflow and are introduced in !109531 (merged).

Screenshots or screen recordings

Here is the mutation creating and returning an instance runner with some of the fields set by the input.

How to set up and validate locally

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

1. Enable the feature flag locally in a rails console bin/rails c:

   Feature.enable(:create_runner_workflow)

2. Visit http://gdk.test:3000/-/graphql-explorer

3. Run this mutation:

   mutation runnerCreate($input: RunnerCreateInput!) {
     runnerCreate(input: $input) {
       runner {
         ephemeralAuthenticationToken
   
         runnerType
         description
         maintenanceNote
         paused
         tagList
         accessLevel
         locked
         maximumTimeout
         runUntagged
       }
       errors
     }
   }

and some example variables

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

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.