Add mutation for clearing project runner cache

What does this MR do and why?

Currently, clearing a project's runner cache is only possible via the UI. Users have requested that it be available via an API as well. This commit adds a new RunnerCacheClear GraphQL mutation that can be used to clear a project's runner cache. It requires that the current user have admin_pipeline permissions on the project, which is the same permission used in the UI.

Changelog: added

Issue: #390684 (closed)

How to set up and validate locally

1. Pick a project and check its jobs_cache_index in the Rails console. It should be nil or an integer.

2. Run the RunnerCacheClear mutation in graphql-explorer. $projectId must be the Global ID of the project

   mutation RunnerCacheClear($projectId: ProjectID!) {
     runnerCacheClear(input: { projectId: $projectId }) {
       errors
     }
   }

3. Recheck the project and see that its jobs_cache_index has been incremented. If it was nil it should now be 1. If it was an integer it should now be one higher.