feat(api): add appsync client interface

[jvh-aws]

Issue #

Description

General Checklist

• Added new tests to cover change, if needed
• Build succeeds with all target using Swift Package Manager
• All unit tests pass
• All integration tests pass
• Security oriented best practices and standards are followed (e.g. using input sanitization, principle of least privilege, etc)
• Documentation update for the change if required
• PR title conforms to conventional commit style
• New or updated tests include Given When Then inline code documentation and are named accordingly testThing_condition_expectation()
• If breaking change, documentation/changelog update with migration instructions

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
⚠️ Please upload report for BASE (feat/appsync-client@47c9ed6). Learn more about missing BASE report.

Additional details and impacted files

@@                  Coverage Diff                   @@
##             feat/appsync-client    #4217   +/-   ##
======================================================
  Coverage                       ?   66.75%           
======================================================
  Files                          ?     1151           
  Lines                          ?    43817           
  Branches                       ?        0           
======================================================
  Hits                           ?    29248           
  Misses                         ?    14569           
  Partials                       ?        0           

Flag	Coverage Δ
API_plugin_unit_test	68.42% <ø> (?)
AWSPluginsCore	68.48% <ø> (?)
Amplify	47.49% <ø> (?)
Amplify_Foundation_Bridge_unit_test	62.28% <ø> (?)
Amplify_Foundation_unit_test	67.64% <ø> (?)
Analytics_plugin_unit_test	83.43% <ø> (?)
Auth_plugin_unit_test	72.28% <ø> (?)
DataStore_plugin_unit_test	81.80% <ø> (?)
Firehose_plugin_unit_test	53.15% <ø> (?)
Geo_plugin_unit_test	73.39% <ø> (?)
Kinesis_plugin_unit_test	52.17% <ø> (?)
Logging_plugin_unit_test	64.86% <ø> (?)
Predictions_plugin_unit_test	33.89% <ø> (?)
PushNotifications_plugin_unit_test	85.66% <ø> (?)
RecordCache_unit_test	76.40% <ø> (?)
Storage_plugin_unit_test	78.67% <ø> (?)
unit_tests	66.75% <ø> (?)

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[harsh62]

Just FYI, china regions are slightly different example appsync-api.cn-north-1.amazonaws.com.cn