Provide access to the merge time of a merge request via API

For our project, we need to retrieve all the requests merged after a given time. This information is currently not available and we can only retrieve the created_at and updated_at fields (the latest being also no enough as a merge request can be updated after being merged).

Feature request:

• Add a merged_at field when querying merge requests
• Add merged_after to filter merge requested merged after a given time