ControlPlaneURI override configuration option

[sirknightj]

Issue #, if available:

• N/A

Description of changes:

• Add an environment variable option to override the control plane URI, same as we have in the https://github.com/awslabs/amazon-kinesis-video-streams-webrtc-sdk-js
• If not provided, it is nil and uses the default endpoint (based on the region)

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[YPandas]

Looks good to me.