Add support for managed identities

Currently the Log Analytics workspace id and key must be provided.

Using a Managed Identity to retrieve this information would be even better.

[Reference](https://docs.microsoft.com/en-us/azure/active-directory/managed-identities-azure-resources/how-to-use-vm-token#get-a-token-using-http)

[Identity readme](https://docs.microsoft.com/en-us/dotnet/api/overview/azure/identity-readme)