Merged filter added to rest api

What does this MR do and why?

this mr add merged_after and merged_before parameter to merge_request rest api

Screenshots or screen recordings

screenrecord

How to set up and validate locally

• pull branch locally
• start gdk
• run this cmd: curl --request GET --header "PRIVATE-TOKEN: gl............." "http://gdk.test:3000/api/v4/projects/2/merge_requests?merged_before=2020-09-03&merged_after=2020-08-03"

related issue: #383512