Enable obtaining authorization tokens for ACR

[sajayantony]

ACR login currently does a handshake with the ACR oauth2 provider. The token is then used to login on the machine which runs the command. In cases when this credential has to be handed off as a secret into another VM or K8s cluster, we want to be able to generate this token without actually doing a docker login.

az acr credential get-authorization-token

Optionally pass in the scope or an an existing access token which can be used to obtain a token based on the access token passed in. For e.g. using MSI.
We also need to consider extending this command to support generating tokens for SAS kind of scenarios.

/cc @DavidObando @shhsu

[sajayantony]

Closing this since we will be enabling a whole token for for ACR.