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

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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 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
           }
         }
       }
     }
   }

ee/app/graphql/resolvers/ci/runner_google_cloud_provisioning_steps_resolver.rb

@@ -18,6 +18,7 @@ class RunnerGoogleCloudProvisioningStepsResolver < BaseResolver
         description: 'Name of the region to provision the runner in.'
       argument :runner_token, ::GraphQL::Types::String,
         required: false,
+        default_value: nil,
         description: 'Authentication token of the runner.'
       argument :zone, ::Types::GoogleCloud::ZoneType,
         required: true,