Handle registration for runners created in GitLab UI

What does this MR do?

This MR prepares gitlab-runner register command to accept a glrt--prefixed authentication token in place of a registration token. In that case, it will call VerifyRunner instead of RegisterRunner and write out the new configuration.

Why was this MR needed?

To cover the plan laid out in the blueprint to accept an authentication token in order to minimize the changes that users need to make to their setup.

What's the best way to test this MR?

https://www.loom.com/share/417024c21ed4430bb5369880a0531fce

An up-to-date GDK server is needed to test this MR, but in case you don't have that, here are the steps:

1. Enable the feature flag in the Rails console:

   Feature.enable(: create_runner_workflow)

2. Create a runner in the GDK (UI for it is not yet available, so you'll need to use the Rails console gdk console):

   Ci::Runners::CreateRunnerService.new(user: User.find(1), type: nil, params: {}).execute.payload[:runner].token

3. Register the runner given the new style glrt- runner token obtained from gdk.test:

   gitlab-runner register -config ~/.gitlab-runner/config.gdk.toml --non-interactive --executor "docker" --docker-image "ubuntu:latest" --url "http://gdk.test:3000/" --description "project test runner" --registration-token="glrt-Mxs8FE84XoupAykHFEiP"

   This results in the following section being added to the config:

   Generated section of config.toml

   
   [[runners]]
     name = "project test runner"
     url = "http://gdk.test:3000/"
     id = 1002
     token = "glrt-Mxs8FE84XoupAykHFEiP"
     token_obtained_at = 2023-02-17T10:04:48Z
     token_expires_at = 0001-01-01T00:00:00Z
     executor = "docker"
     [runners.cache]
       MaxUploadedArchiveSize = 0
     [runners.docker]
       tls_verify = false
       image = "ubuntu:latest"
       privileged = false
       disable_entrypoint_overwrite = false
       oom_kill_disable = false
       disable_cache = false
       volumes = ["/cache"]
       shm_size = 0

4. The registration with a regular registration token should also work as before.

What are the relevant issue numbers?

#29613 (closed)