Add Duo quick action to enable AI in issue/MR/commit/epic workflows.

If we added a /duo quick action it might help with adoption and elevate manual tasks, especially when combined with templates.

examples:

• /duo Summarize the blockers of this epic (#234234) and suggest next steps to address the most pressing one. If there are no blockers do not say anything.
• /duo Create a markdown table with the headings a and b with values [1, 4, 5, 4, 9, 8].
• /duo generate a link to my most recent comment in issue https://gitlab.com/gitlab-org/gitlab/-/issues/511014.

(The last example almost works but fails because Duo doesn't get access to comment ids when it pulls the issue but I see that as an easily fixable issue with Duo and not this proposal).

Issues

When you use a quick action you don't get to preview the response so it will just create the comment. I think this is a workable MVP because the user can then edit the comment to make necessary changes.

I'm not sure what the "killer use cases" are for this but I think it would help with our "Use Duo everywhere" goals.