Support retrieving access tokens from IMDS

Since it appears that Active Directory is the recommended way of controlling access to Azure Storage, it would be useful if azure-storage-python, when running on an Azure Virtual Machine, supported automatically retrieving access tokens from the Azure Instance Metadata Service (IMDS).

I'm willing to take a stab at implementing this functionality if it is something that would be accepted. The biggest question in my mind is - what would the API for enabling this mode look like in the constructor for, say, BaseBlobService.

Start with the BaseBlobService constructor and the Azure Instance Metadata Service (IMDS) access-token flow described in the issue. Determine how this mode should be enabled and how tokens should be retrieved when running on an Azure Virtual Machine; done means the API and implementation support Active Directory access to Azure Storage.

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