Add AI summary on a new merge request screen

Following up from https://gitlab.com/gitlab-org/gitlab/-/issues/429792

The current way we display the Ai-generated summary is getting duplicated by the human-generated description. The use case where we get a real value is when authors don't provide a description and the AI creates a correct summary, but currently the AI isn't getting 100% correctness on the summary, there is a lot of distrust from viewers of the MR, some users mentioned that they would feel better if they could see that the summary was reviewed and approved by a human.

We should provide a way for authors to generate their summaries when creating the MR, right on the creation/edit screen. So they can decide if they want to use the summary as the description of the MR and even if the AI is not 100% correct the author can edit the text and make it their own.