Add enum defining Azure clouds #41846
Description
Add an enum, or ExpandableStringEnum, which defines the well-known Azure clouds supported by azure-core.
In addition to the enum, add support for reading an environment variable to set the value being used, initial proposal is AZURE_CLOUD. If the value is set to a valid enum value, use that, otherwise default to the Azure public cloud.
This enum will need to be passed around in a few locations to modify behaviors to use the correct cloud information. This issue will be updated once that list is compiled.