Support Azure Container Instance using managed identity for auth

#### Description

Using a managed identity for auth is supported an Azure VMs.
It would be very valuable if Container Instances would be supported too. As Container Instances are more lightweight, and designed for one-time build tasks, they are actually a better platform for building packer images than VMs.

#### Use Case(s)

Run an Azure Container Instance with a managed id, and execute a packer build to create an image

#### Potential configuration

Same as for VMs (as described in docs).

The issue does not name files or tests; start by tracing the existing Azure VM managed-identity authentication described in the documentation. Determine how that configuration could apply to Azure Container Instances, then verify that a Container Instance can run a Packer build using a managed identity.

Written by the indexing model from the issue text.