Add runner registration feedback

What does this MR do and why?

This change adds feedback for the user to see when a runner registration has been successful.

Screenshots or screen recordings

See the entire demo at: https://youtu.be/6Bi8XsB73_M

How to set up and validate locally

1. Enable the feature flag: in your console rails c, and then Feature.enable(:create_runner_workflow_for_admin)
2. Use the main version of runner (note that this version has not been released)

$ cd ~
$ curl --output gitlab-runner https://gitlab-runner-downloads.s3.amazonaws.com/main/binaries/gitlab-runner-darwin-amd64
$ chmod +x gitlab-runner
$ ./gitlab-runner install

2. Follow the runner creation instructions from http://gdk.test:3000/admin/runners

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #383139 (closed)