Show HCL variable input on job submission.

[sevensolutions]

Description

This PR will show the text area for entering HCL variable values on job submission, as suggested in #23582.
It was already shown when editing a job using the UI, but not when submitting a new job.
The underlying model already supports the this, only the UI was missing.

Testing & Reproduction steps

• Submit a new job using variables
• Enter the variables in the new text area below the job spec

Links

• Github Issue: Add variables text box on job submission page #23582

Contributor Checklist

• Changelog Entry If this PR changes user-facing behavior, please generate and add a
  changelog entry using the make cl command.
• Testing Please add tests to cover any new functionality or to demonstrate bug fixes and
  ensure regressions will be caught.
• Documentation If the change impacts user-facing functionality such as the CLI, API, UI,
  and job configuration, please update the Nomad website documentation to reflect this. Refer to
  the website README for docs guidelines. Please also consider whether the
  change requires notes within the upgrade guide.

Reviewer Checklist

• Backport Labels Please add the correct backport labels as described by the internal
  backporting document.
• Commit Type Ensure the correct merge method is selected which should be "squash and merge"
  in the majority of situations. The main exceptions are long-lived feature branches or merges where
  history should be preserved.
• Enterprise PRs If this is an enterprise only PR, please add any required changelog entry
  within the public repository.

[philrenaud]

@sevensolutions this looks and works great. Nice job and thanks for the contribution!

Would you mind adding a changelog entry by running make cl? With that, I would be happy to see this merged.