Add grading standard to assignment type

refs VICE-3774
flag=student_grade_summary_upgrade

Test Plan:
- add a custom grading scheme
- create an assignment
 - set display points as "Letter Grade"
 - add the custom grading scheme to the assignment
- should be able to query the grading standard
   on the assignment type

Sample query:
query MyQuery {
  legacyNode(_id: "1019", type: Course) {
    ... on Course {
      id
      name
      _id
      assignmentsConnection {
        nodes {
          _id
          name
          gradingStandard {
            id
            title
            data {
              baseValue
              letterGrade
            }
          }
        }
      }
    }
  }
}

Change-Id: I65c08791a30f45900e982dc4f5eae21a532cb8fe
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/328863
Reviewed-by: Omar Soto-Fortuño <omar.soto@instructure.com>
Product-Review: Omar Soto-Fortuño <omar.soto@instructure.com>
QA-Review: Caleb Guanzon <cguanzon@instructure.com>
Tested-by: Service Cloud Jenkins <svc.cloudjenkins@instructure.com>

app/graphql/types/assignment_type.rb

@@ -467,6 +467,11 @@ module Types
       SubmissionSearch.new(assignment, current_user, session, filter).search
     end
 
+    field :grading_standard, GradingStandardType, null: true
+    def grading_standard
+      load_association(:grading_standard)
+    end
+
     field :group_submissions_connection, SubmissionType.connection_type, null: true do
       description "returns submissions grouped to one submission object per group"
       argument :filter, SubmissionSearchFilterInputType, required: false

spec/graphql/types/assignment_type_spec.rb

@@ -307,6 +307,19 @@ describe Types::AssignmentType do
     expect(assignment_type.resolve("allowedAttempts")).to eq 7
   end
 
+  describe "gradingStandard" do
+    it "returns the grading standard" do
+      grading_standard = course.grading_standards.create!(title: "Win/Lose", data: [["Winner", 0.94], ["Loser", 0]])
+      assignment.update(grading_type: "letter_grade", grading_standard_id: grading_standard.id)
+      assignment.save!
+      expect(assignment_type.resolve("gradingStandard { title }")).to eq grading_standard.title
+    end
+
+    it "returns null if no grading standard is set" do
+      expect(assignment_type.resolve("gradingStandard { title }")).to be_nil
+    end
+  end
+
   describe "submissionsConnection" do
     let_once(:other_student) { student_in_course(course:, active_all: true).user }