feat(core,api): IAM auth mode for HTTP requests (REST and GQL)

[ragingsquirrel3]

This PR does the following:

• migrates logic for API key auth mode to auth provider, including definition in core
• consumes IAM auth provider in API plugin for HTTP requests, the result being that REST and GraphQL queries/mutations will work with IAM auth mode. I plan to have integration tests for this in another PR.
• unit tests for that stuff

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[codecov-commenter]

Codecov Report

Merging #1893 (4252d70) into feat/api-next (7a82e9e) will increase coverage by 9.57%.
The diff coverage is 96.92%.

@@                Coverage Diff                @@
##           feat/api-next    #1893      +/-   ##
=================================================
+ Coverage          35.92%   45.49%   +9.57%     
=================================================
  Files                116      129      +13     
  Lines               6513     7697    +1184     
=================================================
+ Hits                2340     3502    +1162     
- Misses              4173     4195      +22     

Flag	Coverage Δ
android-unit-tests	∅ <ø> (∅)
flutter-unit-tests	29.26% <96.92%> (+0.98%)	⬆️
ios-unit-tests	89.08% <ø> (+8.87%)	⬆️

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

Impacted Files	Coverage Δ
...api/lib/src/decorators/authorize_http_request.dart	95.45% <95.45%> (ø)
...api/lib/src/amplify_authorization_rest_client.dart	62.50% <100.00%> (+62.50%)	⬆️
...kages/api/amplify_api/lib/src/api_plugin_impl.dart	88.11% <100.00%> (+11.98%)	⬆️
...ib/src/graphql/app_sync_api_key_auth_provider.dart	100.00% <100.00%> (ø)
...mple/ios/unit_tests/QueryPaginationUnitTests.swift	100.00% <0.00%> (ø)
...os/unit_tests/ModelSchemaEquatableExtensions.swift	89.83% <0.00%> (ø)
...mple/ios/unit_tests/DataStorePluginUnitTests.swift	96.81% <0.00%> (ø)
.../example/ios/unit_tests/resources/SchemaData.swift	100.00% <0.00%> (ø)
...it_tests/DataStoreHubEventStreamHandlerTests.swift	97.87% <0.00%> (ø)
...e/ios/unit_tests/AmplifyModelSchemaUnitTests.swift	97.70% <0.00%> (ø)
... and 7 more

[Equartey]

Looks good, nice work!