Support cost factor fields in runner creation GraphQL

What does this MR do and why?

Support cost factor fields in runner creation GraphQL

In favor of managing GitLab.com Hosted Runners through Terraform, we must make cost factor fields editable with an API. That's a requiremet for making the GitLab Provider for Terraform able to handle that.

This commit adds needed GraphQL updates to support the fields in API.

Related to Manage Hosted Runners tokens through Terraform (gitlab-org/ci-cd/shared-runners/infrastructure#263) • Unassigned

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Before	After

How to set up and validate locally

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

1. Run the following mutation on http://gdk.test:3000/-/graphql-explorer:

   mutation runnerUpdate($input: RunnerUpdateInput!) {
     runnerUpdate(input: $input) {
       runner {
         id
         runnerType
         privateProjectsMinutesCostFactor
         publicProjectsMinutesCostFactor
       }
       errors
     }
   }

   Variables (pick the ID of an existing runner with puts Ci::Runner.last.to_global_id):

   {
     "input": {
       "id": "gid://gitlab/Ci::Runner/1053",
       "privateProjectsMinutesCostFactor": 0.5
     }
   }