Add platforms options to new runner layout

What does this MR do and why?

This change adds platform options to the runner creation form, so users can define which platform (OS, cloud provider, container) they use to create their runner.

Designs: #383139[Runner-registration-page.png]

Note: Only the first "platforms" field is added in this MR, subsequent MR will add the rest of the fields and the submit button. The feature is being built behind: create_runner_workflow

Screenshots or screen recordings

Admin -> Runners -> New instance runner e.g. http://gdk.test:3000/admin/runners/new

How to set up and validate locally

1. Enable the feature flag: in your console rails c, and then Feature.enable(:create_runner_workflow)
2. Visit Admin -> CI/CD -> Runners
3. Confirm the "New instance runner" button is available on the top-right corner
4. Select the button to go to the new page

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)