refactor: update assessments formats

[nsprenkle]

TL;DR - Update assessments data format used to communicate with frontend.

JIRA: AU-1528 and AU-1525 (closes #2103 )

What changed?

Frontend now uses this format to communicate assessments data to / from the backend:

{
    criteria: [
        // Rubric criterion
        {
            selectedOption: (Int) Order of the selected option
            feedback: (String / Empty) Feedback for the selected option
        }
        ...
    ],  
    overallFeedback: (String / Empty)
}

Included supporting changes to convert this to the format used by the backend to submit an assessment.

Reviewer Checklist

Collectively, these should be completed by reviewers of this PR:

• I've done a visual code review
• I've tested the new functionality

FYI: @openedx/content-aurora

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (8419f5e) 94.97% compared to head (c86e7be) 94.98%.
Report is 1 commits behind head on feat/2023-dual-ui.

Additional details and impacted files

@@                  Coverage Diff                  @@
##           feat/2023-dual-ui    #2106      +/-   ##
=====================================================
+ Coverage              94.97%   94.98%   +0.01%     
=====================================================
  Files                    191      191              
  Lines                  20383    20448      +65     
  Branches                1848     1851       +3     
=====================================================
+ Hits                   19359    19423      +64     
- Misses                   769      770       +1     
  Partials                 255      255              

Flag	Coverage Δ
unittests	94.98% <100.00%> (+0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
...ent/xblock/ui_mixins/mfe/assessment_serializers.py	100.00% <100.00%> (ø)
openassessment/xblock/ui_mixins/mfe/mixin.py	94.66% <100.00%> (+0.02%)	⬆️
...block/ui_mixins/mfe/test_assessment_serializers.py	100.00% <100.00%> (ø)
...nassessment/xblock/ui_mixins/mfe/test_mfe_mixin.py	99.79% <ø> (ø)

... and 3 files with indirect coverage changes

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