azure: add support for service principals - fixes #3230

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Before: users can only connect to Azure blob containers using the access keys
from the storage account.

After: users can additionally choose connect to Azure blob containers
using service principals. This uses OAuth2 under the hood to exchange
a client ID and client secret for a short-lived access token.

Ref:
- https://github.com/rclone/rclone/issues/3230
- https://docs.microsoft.com/en-us/azure/storage/common/storage-auth-aad-app?tabs=dotnet#well-known-values-for-authentication-with-azure-ad
- https://docs.microsoft.com/en-us/azure/developer/go/azure-sdk-authorization#available-authentication-types-and-methods
- https://gist.github.com/ItalyPaleAle/ec6498bfa81a96f9ca27a2da6f60a770

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