WIP: Resolve ""New branch" slash command"

What does this MR do?

It adds an extra slash command "/branch" to the issue comment section.

Are there points in the code the reviewer needs to double check?

The DSL for slash commands does not have a way for params to be optional, even though the command_definition_spec.rb file has a test for it. I have added in a new DSL keyword optional_params in CommandDefinition class to allow for this.

Why was this MR needed?

Resolution for issue #27801 (moved).

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #27801 (moved)