Expose `canMerge` for GraphQL MR author and participants

What does this MR do and why?

Exposes the permissions of the participating users in graphql

Graphql query

query getMrParticipants($fullPath: ID!, $iid: String!) {
  project(fullPath: $fullPath) {
    id
    mergeRequest(iid: $iid) {
      id
      participants {
        nodes {
          id
          name
          mergeRequestInteraction {
            canMerge
          }
        }
      }
      author {
        name
        id
        mergeRequestInteraction {
            canMerge
        }
      }
    }
  }
}

Related to #353982 (closed)