Export merge requests to CSV like already possible for issues

Description

I want to be able to export merge requests to CSV like possible for issues.

Solution

On the Merge Requests index, add a button to "Export as CSV" (like the one on the Issues index). Clicking this button would cause a CSV file to be downloaded with these columns:

CSV schema

Column	Description
MR ID	MR iid
URL	A link to the merge request on GitLab
Title	Merge request title
State	Opened, Closed, Locked, or Merged
Description	Merge request description
Source Branch	Source branch
Target Branch	Target branch
Source Project ID	ID of the source project
Target Project ID	ID of the target project
Author	Full name of the merge request author
Author Username	Username of the author, with the @ symbol omitted
Assignees	Full names of the merge request assignees, joined with a ,
Assignee Usernames	Username of the assignees, with the @ symbol omitted
Approvers	Full names of the approvers, joined with a ,
Approver Usernames	Username of the approvers, with the @ symbol omitted
Merged User	Full name of the merged user
Merged Username	Username of the merge user, with the @ symbol omitted
Milestone ID	ID of the merge request milestone
Created At (UTC)	Formatted as YYYY-MM-DD HH:MM:SS
Updated At (UTC)	Formatted as YYYY-MM-DD HH:MM:SS

UI

Links / references

https://docs.gitlab.com/ee/user/project/issues/csv_export.html

Feature checklist

Make sure these are completed before closing the issue, with a link to the relevant commit.

• Feature assurance
• Documentation
• Added to features.yml