upgrade get_grading_config api with DRF

[awais786]

existing API code

Things to implement.

1. Use DRF to upgrade this API.
2. Use the standard edx-platform authentication.
3. Maintain existing behaviour and status codes.

verify with postman

1: Use this URL with post request. ( pass your valid course id )

expected result
{ "course_id": "course-v1:edx+cs222+2311", "grading_config_summary": "<pre>-----------------------------------------------------------------------------\nCourse grader:\n&lt;class &#39;xmodule.graders.WeightedSubsectionsGrader&#39;&gt;\n\nGraded sections:\n subgrader=&lt;class &#39;xmodule.graders.AssignmentFormatGrader&#39;&gt;, type=Homework, category=Homework, weight=0.15\n subgrader=&lt;class &#39;xmodule.graders.AssignmentFormatGrader&#39;&gt;, type=Lab, category=Lab, weight=0.15\n subgrader=&lt;class &#39;xmodule.graders.AssignmentFormatGrader&#39;&gt;, type=Midterm Exam, category=Midterm Exam, weight=0.3\n subgrader=&lt;class &#39;xmodule.graders.AssignmentFormatGrader&#39;&gt;, type=Final Exam, category=Final Exam, weight=0.4\n subgrader=&lt;class &#39;xmodule.graders.AssignmentFormatGrader&#39;&gt;, type=Entrance Exam, category=Entrance Exam, weight=0.0\n-----------------------------------------------------------------------------\nListing grading context for course course-v1:edx+cs222+2311\ngraded sections:\n[&#39;Entrance Exam&#39;]\n--&gt; Section Entrance Exam:\n Entrance Exam - Subsection (format=Entrance Exam, Assignment=Entrance 01)\nall graded blocks:\nlength=1\n</pre>" }

end to end testing

1. Go to instructor page and data download tab. Using tutor use this link ( your valid course id )
2. Press Grading Configuration button.