Duo Workflow in Remote Execution should optionally open an MR

Problem

Right now Duo Workflow in remote execution is just editing code in a CI job which is thrown away. After #516132 (closed) we'll be pushing commits to a branch.

But many workflows should end with an MR for the user to review.

Solution

We should add somewhere to the user flow the ability to signify that the workflow should create an MR. We can then either prompt the LLM to do this themselves via tool calls or we can explicitly hard code this in Duo Workflow Service to open the MR for the workflow the at the end. The former approach is more flexible but likely less reliable. If we have specific uses where the user has explicitly asked for an MR it seems easier to just code that explicitly into the Duo Workflow Service and not rely on the LLM's judgement.