Support Azure Managed Identity authentication to azure

[daviditkin]

conf.json supports the managed-identity boolean property. When true, managed identity authentication will be used.

Refactored authentication logic out of RestClient into Authenticator along with AuthStrategy(s) including ManagedIdentityAuthStrategy. I did this because I didn't want to add more if/else logic to the RestClient than is already there.

GenericAuthStrategy is just the original collection of authentication logic that already existed. It could be broken up into additional AuthStrategy types.

Token was modified to handle either int or string integer json values for the expiration times. See IntOrStringInt.

[daviditkin]

recheck