feat(api): .subscribe() for GraphQL

[ragingsquirrel3]

WIP: This is a rough early draft of subscriptions for GraphQL in dart. It implements the appsync real-time client described on https://docs.aws.amazon.com/appsync/latest/devguide/aws-appsync-real-time-data.html. It still needs tests, error handling, comments, cleanup, etc...

Just trying to solicit some early feedback on the approach before adding 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 #1915 (6e384c3) into feat/api-next (3ac59f4) will increase coverage by 0.22%. The diff coverage is 71.92%.

@@                Coverage Diff                @@
##           feat/api-next    #1915      +/-   ##
=================================================
+ Coverage          44.82%   45.05%   +0.22%     
=================================================
  Files                129      129              
  Lines               7752     7853     +101     
=================================================
+ Hits                3475     3538      +63     
- Misses              4277     4315      +38     

Flag	Coverage Δ
android-unit-tests	∅ <ø> (∅)
flutter-unit-tests	30.00% <71.92%> (+1.50%)	:arrow_up:
ios-unit-tests	89.48% <ø> (+0.39%)	:arrow_up:

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

Impacted Files	Coverage Δ
...kages/api/amplify_api/lib/src/api_plugin_impl.dart	81.73% <30.00%> (-5.51%)	:arrow_down:
..._api/lib/src/graphql/ws/web_socket_connection.dart	59.77% <59.77%> (ø)
...plify_api/lib/src/graphql/ws/web_socket_types.dart	79.36% <79.36%> (ø)
...phql/ws/web_socket_message_stream_transformer.dart	90.47% <90.47%> (ø)
..._api/lib/src/decorators/web_socket_auth_utils.dart	100.00% <100.00%> (ø)
.../ios/unit_tests/amplify_flutter_exampleTests.swift
...plify_flutter/example/ios/Runner/AppDelegate.swift
...ter/example/ios/unit_tests/AtomicResultTests.swift
...e/ios/unit_tests/MockAnalyticsCategoryPlugin.swift

[ragingsquirrel3]

I'm still working on this, especially testing, logging, error handling, many comments not addressed. However, I did refactor the auth logic to use more pure functions and added lots of comments there. Let me know if still difficult to follow.

[ragingsquirrel3]

Nice work!

Question: Does the example app need to be updated to reflect any api changes?

No API changes for subscriptions, though I did just make a change to example app with last rev so unsubscribe button works.