Deprecate or enhance azure plugin

azure plugin only supports Service Principal with password authentication. This is regarded insecure to authenticate with Azure. It is now recommended to use Managed Identity or SP with certificate for authentication. Moreover, the password is specified in the coredns config as plain text, which incurs a secret leakage concern. The plugin also indirectly imports a deprecated package (github.com/Azure/go-autorest/autorest/adal) for Azure authentication. The package is no longer supported for 1+ year.

I'm wondering what's the adoption of this plugin. We should either enhance it or deprecate it.