Task Add Merge Train Car to MergeRequestType

What does this MR do and why?

This MR adds :merge_train_car to MergeRequestType in the graphql schema.

• field returns nil if merge trains not enabled for the MR's project
• field resolves with associated merge_train_car

Query example

     query getMergeRequest($gid: MergeRequestID!){
       mergeRequest(id: $gid){
         mergeTrainCar{
           id
         }
       }
     }

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Enable merge trains for a project
2. Create a few MRs and add them to a merge train
3. navigate to graphiql /-/graphql-explorer
4. Test with query example(above) using local merge request

Related to #471112 (closed)

Related to #471631 (closed)