Support Managed Identity auth for the azure-arm builder

Support Managed Identity auth for the azure-arm builder: a task in MiMo-V2.6-RL-harbor-code: MiMo-V2.6-RL Code (Harbor) (Harbor dataset). I'd like to run Packer with the azure-arm builder from inside an Azure VM that has a system-assigned managed identity, and have Packer use that identity to talk…