Design: The new "prefill inputs" page on the Run Pipeline page

Summary

As a part of Add support for spec:inputs in GitLab CI pipelines (&16321 - closed); implement the new prefill inputs page. (scope: only the Run Pipeline page. The "Schedule a new pipeline" page will be handled in a different issue)

• Required backend; #519944 (closed)
• Some information about prefill inputs/variables; &16321 (comment 2353206073)

Proposal

• Figma
• Design tab

Out of scope of this issue

• Form validation based on the spec:inputs:type: Design: Add form validations for the prefill in... (#521003 - closed)
  • Except for the type: number.
• Advanced styling