Add enum defining Azure clouds

[alzimmermsft]

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.

[XiaofeiCao]

new AzureCloud enum, tasks:

• AzureCloud in azure-core
• adopt AzureCloud in AzureProfile from azure-core-maangement
• update SDK documentation to adopt the new AzureProfile ctor overload
• adopt potential new azure-identity API with AzureCloud support
• (Not planned) new AZURE_CLOUD environment variable

[srnagar]

This is done in both azure-core and clientcore
azure-core - #43099
clientcore - #44301