feat(all): Add ability to use OkHttp4 with Amplify v2.x

[mattcreaser]

• PR title and description conform to Pull Request guidelines.

Issue #, if available:
#2809

Description of changes:
All AWS clients will now use the Http4Engine if it is available on the classpath.

How did you test these changes?

• Create test app using OkHttp4 and verify the correct engine is used in Amplify

Documentation update required?

• No
• Yes: Docs update TODO

General Checklist

• Added Unit Tests
• Added Integration Tests
• Security oriented best practices and standards are followed (e.g. using input sanitization, principle of least privilege, etc)

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

[tylerjroach]

I checked for all extensions of SdkClient and verified that no use cases were missed.

Can you confirm that our own usages of OkHttp don't use any OkHttp v5 only methods. Examples of these are the OkHttp clients that we create within Amplify API and our Liveness Rekognition WebSocket.

[tylerjroach]

Tested with Liveness Sample app using both API and Liveness OkHttp v4 and all worked as expected.