Add group runners creation and registration pages

What does this MR do and why?

This change adds the runner create and register pages for the new runner creation workflow behind flag: create_runner_workflow_for_namespace.

The change will allow group administrators to add their runners using the new workflow.

This is a large MR!

Yes, this MR covers quite a bit of ground as it implements most of #383143 (closed) in a single step. However, I think it makes sense to build it like this:

• The feature flag create_runner_workflow_for_namespace is disabled by default
• This feature was already built gradually for admin, so there is no need to split this implementation over several steps.
• The designs/UX flow is the same as the admin usage
• This MR allows us to test the entire flow

Screenshots or screen recordings

Step 1: Create runner	Step 2: Register runner

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_namespace)

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

3. Follow the runner creation instructions in a group your manage (as group Owner) from http://gdk.test:3000/groups/-/runners/new

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 #383143 (closed)