chore(datastore): Migrate `ModelAssociation`

[dnys1]

Migrates ModelAssociation and ModelAssociationEnum to the new MIPR format.

Internally breaking changes:

• Renames ModelAssociationEnum types to camelCase
• ModelAssociationEnum has been renamed to ModelAssociationType

---

Stack:

• #2170 ⬅
• #2159
• #2158

⚠️ Part of a stack created by spr. Do not merge manually using the UI - doing so may have unexpected results.

[codecov-commenter]

Codecov Report

Merging #2170 (2ed32f1) into pr/dnys1/chore/next/datastore/migration/7a36c482 (704a44d) will increase coverage by 0.01%. The diff coverage is 100.00%.

@@                                 Coverage Diff                                  @@
##           pr/dnys1/chore/next/datastore/migration/7a36c482    #2170      +/-   ##
====================================================================================
+ Coverage                                             42.86%   42.88%   +0.01%     
====================================================================================
  Files                                                   114      113       -1     
  Lines                                                  7502     7499       -3     
====================================================================================
  Hits                                                   3216     3216              
+ Misses                                                 4286     4283       -3     

Flag	Coverage Δ
android-unit-tests	∅ <ø> (∅)
flutter-unit-tests	25.08% <100.00%> (ø)
ios-unit-tests	89.19% <ø> (+0.12%)	:arrow_up:

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

Impacted Files	Coverage Δ
...ackages/api/amplify_api/lib/src/graphql/utils.dart	91.52% <100.00%> (ø)
..._pinpoint_ios/example/ios/Runner/AppDelegate.swift