Use DefaultAzureCredentials for workspace auth

Apparently, it is not possible to use DefaultAzureCredentials for workspace auth.

From the [docs](https://github.com/Azure/MachineLearningNotebooks/blob/master/how-to-use-azureml/manage-azureml-service/authentication-in-azureml/authentication-in-azureml.ipynb), the available auth types are:
Interactive Login Authentication
Azure CLI Authentication
Managed Service Identity (MSI) Authentication
Service Principal Authentication
Token Authentication

It would be very useful if this SDK used the standard Azure way to handle authentication

Start with the authentication-in-azureml.ipynb documentation linked in the issue and review its listed workspace authentication paths. Compare those paths with the requested DefaultAzureCredentials support; done means the standard Azure credential path is supported and documented for workspace authentication.

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