Allow override of endpoint suffixes with env vars

[jasonpnnl]

Closes #375

This adds the ability for endpoint suffixes to be overridden using environment variables instead of them being hardcoded to the suffixes used by US regions. This is helpful when using sovereign clouds or in other cases where the endpoint suffix is not the default without having to make modifications to the hardcoded values. The hardcoded values are kept as defaults for backward compatibility.

[jasonpnnl]

@microsoft-github-policy-service agree company="PNNL"