Add support for AWS China partition

[ansromanov]

Add support to log in to AWS China accounts with Okta.

Example log:

INFO - App Link set as: https://xxxxxx.okta.com/home/amazon_aws/xxxxxx
INFO - Authenticating to: xxxxxx.okta.com
INFO - Authenticating as: [email protected]
Multi-factor Authentication required for application.
Pick a factor:
[ 0 ] Okta Verify App: SmartPhone_Android: XXXXXXX
[ 1 ] token:software:totp( OKTA ) : [email protected]
Selection: 0
INFO - Okta Verify push sent...
INFO - Using predefined role: arn:aws-cn:iam::111111111111:role/DeveloperRole
INFO - Authenticating to: xxxxxx.okta.com
INFO - Session token expires on: 2021-11-17 14:37:24+00:00
Use these to set your environment variables:
export AWS_ACCESS_KEY_ID=XXXX
export AWS_SECRET_ACCESS_KEY=XXXX
export AWS_SESSION_TOKEN=XXXX```

[TryTryAgain]

Fantastic, it would be great if this could be merged!