GraphQL: Create group and project runners in runnerCreate mutation

What does this MR do and why?

Describe in detail what your merge request does and why.

This MR adapts the runnerCreate mutation introduced in !110593 (merged) to be able to create group and project runners, in addition to the instance runners that it was able to create.

The supporting service was adapted accordingly.

Closes #393889 (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. Pick a group from http://gdk.test:3000/dashboard/groups and note its database ID (let's say it is ID 24):

   

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

   Feature.enable(:create_runner_workflow_for_namespace, Group.find(24))

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

4. Run this mutation:

   mutation runnerCreate($input: RunnerCreateInput!) {
     runnerCreate(input: $input) {
       runner {
         ephemeralAuthenticationToken
   
         runnerType
         description
         maintenanceNote
         paused
         tagList
         accessLevel
         locked
         maximumTimeout
         runUntagged
   
         #############################
         # if runnerType == GROUP_TYPE
         groups {
           nodes {
             id
           }
         }
         #############################
   
         #############################
         # if runnerType == PROJECT_TYPE
         ownerProject {
           id
         }
         #############################
       }
       errors
     }
   }

   and some example variables (use the ID from the group above in groupId):

   {
     "input": {
       "runnerType": "PROJECT_TYPE",
       "groupId": "gid://gitlab/Group/24",
       "description": "my awesome description",
       "maintenanceNote": "often.",
       "paused": true
     }
   }

MR acceptance checklist

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