Add API pagination for deployed merge requests

This endpoint did not apply pagination to the output, which can result
in API calls taking a long time when many deployments are retrieved.

See https://gitlab.com/gitlab-org/gitlab/issues/207523 for more
information.

changelogs/unreleased/fix-deployments-pagination.yml

+---
+title: Add API pagination for deployed merge requests
+merge_request: 25733
+author:
+type: performance

lib/api/deployments.rb

@@ -143,6 +143,7 @@ module API
         success Entities::MergeRequestBasic
       end
       params do
+        use :pagination
         requires :deployment_id, type: Integer, desc: 'The deployment ID'
         use :merge_requests_base_params
       end
@@ -153,7 +154,7 @@ module API
         mr_params = declared_params.merge(deployment_id: params[:deployment_id])
         merge_requests = MergeRequestsFinder.new(current_user, mr_params).execute
 
-        present merge_requests, { with: Entities::MergeRequestBasic, current_user: current_user }
+        present paginate(merge_requests), { with: Entities::MergeRequestBasic, current_user: current_user }
       end
     end
   end

spec/requests/api/deployments_spec.rb

@@ -444,6 +444,7 @@ describe API::Deployments do
         subject
 
         expect(response).to have_gitlab_http_status(:ok)
+        expect(response).to include_pagination_headers
         expect(json_response.map { |d| d['id'] }).to contain_exactly(merge_request1.id, merge_request2.id)
       end