GraphQL: Use query field instead of mutation

What does this MR do and why?

This MR is a follow-up to !145598 (comment 1789427546) to replace the provisionGoogleCloudRunner mutation with the provisioningSteps query field. It also renames runnerCloudProvisioningOptions to runnerCloudProvisioning, since this is no longer only about options.

EE: true

Closes #438316 (closed)

Review status

Reviews	Reviewer	Status
backend (initial)	@10io	✅
backend (maintainer)	@dmeshcharakou	✅
frontend (maintainer)	@mrincon	✅
devopsverify	@vshushlin	✅
authorization	@jarka	✅
groupimport and integrate	@bmarjanovic	✅
documentation	@fneill	✅

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

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 query in http://gdk.test:3000/-/graphql-explorer:

   {
     project(fullPath: "gitlab-org/playground") {
       id
       runnerCloudProvisioning(
         provider: GOOGLE_CLOUD
         cloudProjectId: "dev-gcp-s3c-integrati-9abafed1"
       ) {
         ... on CiRunnerGoogleCloudProvisioning {
           provisioningSteps(
             region: "us-central1"
             zone: "us-central1-a"
             ephemeralMachineType: "n2d-standard-2"
             runnerToken: "glrt-Ei-AAaaaaAAAaaAA"
           ) {
             title
             languageIdentifier
             instructions
           }
         }
       }
     }
   }