Resolve "Customize branch name when using create branch in an issue"

What does this MR do?

This MR adds an ability to type a custom branch name in the Create merge request dropdown placed in an issue.

TODO

• Add backend specs
• Add frontend specs

Why was this MR needed?

It improves UX of branch management.

Screenshots (if relevant)

To be added.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?

Closes #21143 (closed)