Add branch selector to on-demand scans form

What does this MR do?

This adds a branch selector input to let user select the branch a DAST scan is associated with.

How to test this?

1. Enable the dast_branch_selection feature flag.

echo "Feature.enable(:dast_branch_selection)" | rails c

2. Navigate to the on-demand scans form at /:namespace/:project/-/on_demand_scans/new.

Screenshots (strongly suggested)

branch

Before	After

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • [-] I have included a changelog entry.
  • I have not included a changelog entry because those changes are behind a disabled feature flag.
• [-] Documentation (if required) #322673 (closed)
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• [-] Merge request performance guidelines
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Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Related to #322872 (closed)