Create runner form: Add the second step with optional fields

What does this MR do and why?

This adds the second step of the Create Runner form. This step consists of optional fields. If none of them are filled in / checked, there will be no error.

This step doesn't create a new runner. Runner creation will be done in another MR to keep MRs easier to review.

References

Feature issue: #396544

FF rollout issue: #533827

UX Paper Cuts planning issue: #512693 (closed), #512695 (closed)

Screenshots or screen recordings

How to set up and validate locally

1. In rails console enable the feature flag

   Feature.enable(: runner_create_wizard_admin)

2. Go to http://gdk.test:3000/admin/runners/new

3. Write a any text in the tag field

4. Click [Next step]

5. Now you should see the second step of the form

MR acceptance checklist

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