Make Azure environments pluggable

[jhendrixMSFT]

It would get ideal to be able to seamlessly switch clouds, i.e. public cloud -> sovereign cloud. In track 1 there is the Environment settings but it requires the caller to wire up all the endpoints themselves.
One idea is to add an Environment arg to client constructors that will contain all the endpoints and scope. It would need to include the authority host to be passed to the GetToken() call.

[RickWinter]

Need to review our EndPoint usage. The SDK needs to follow design guidelines for talking to many clouds

[JeffreyRichter]

@jhendrixMSFT This is a very old issue. I believe that our clients support multiple clouds simply by requiring the customer to pass an endpoint when creating each client and also by allowing them to override the authentication scope. Do you think there is anything else we need to do?

[jhendrixMSFT]

@JeffreyRichter while we do support this today via custom endpoints, you have to match them when authenticating and creating a client. What I was conceptually envisioning was that you could indicate "use USGov cloud" and the correct endpoints would be wired up.

[seankane-msft]

@chlowell has this issue been addressed by the GetToken methods in azidentity?

[chlowell]

No. To use ARM in a sovereign cloud you must at least configure the credential to authenticate in that cloud, and point your ARM client at that cloud's management endpoint. That is to say, we have two separate configuration points you must set with matching values or your application doesn't work. I believe Joel's looking for a way to tell the SDK as a whole "my application is in this cloud" and have it handle the details internally, which would make this a relative of Azure/azure-sdk#1550

[RickWinter]

We need to ensure we can add this in the future without making a breaking change

[jhendrixMSFT]

@chlowell I believe this work is now complete?

[chlowell]

Yes, this was closed by #16942