[FEATURE REQ] Support Custom Cloud in Microsoft.Azure.Management.ResourceManager.Fluent.AzureEnvironment

[bganapa]

Is your feature request related to a problem? Please describe.
Microsoft.Azure.Management.ResourceManager.Fluent.AzureEnvironment has defined a static list of known environments. This class is heavily used when initializing all resourcemanagement clients. This is making it difficult for other custom azure clouds like AzureStack.

Describe the solution you'd like
Suggest adding a method in this class like GetAzureEnvironmentFromArmEndpoint
We have a similar method in Go SDK - https://github.com/Azure/go-autorest/blob/711dde1b1be6e3a7da75975e08f64e0cafb905a3/autorest/azure/metadata_environment.go#L96

All the needed information to initalize the AzureEnvironment can be read from ARM metadata endpoint itself (the existign hardcoded values can be removed as well)
e.g https://management.azure.com/metadata/endpoints?api-version=1.0
https://management.azure.com/metadata/endpoints?api-version=2019-10-01

Describe alternatives you've considered
defining the method ourselves wherever the client needs to be initialized.

Information Checklist
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