This PR add supports for auth method Federated Workload Identity. In order to use this auth method for github, users need to follow up the https:registry.terraform.io/providers/hashicorp/azurerm/latest/docs/guides/service_principal_oidc to setup the SP and also the necessary env var. Then launch the command with option: --use-oidc-cred.

A example workflow can be found at: https:github.com/magodo/aztfy-oidc-test/blob/2eaad2c1f47cf369955fb7a1f715a05488faa48f/.github/workflows/test.yaml. Basically, following settings are required:

permissions:
  id-token: write
  contents: read

jobs:
  test:
    runs-on: ubuntu-latest
      #...
      - name: Export the demo rg using oidc
        run:
          aztfexport rg -n -f -use-oidc-cred --plain-ui my-rg
        env:
          ARM_USE_OIDC: true
          ARM_TENANT_ID: ${{ secrets. ARM_TENANT_ID }}
          ARM_SUBSCRIPTION_ID: ${{ secrets. ARM_SUBSCRIPTION_ID }}
          ARM_CLIENT_ID: ${{ secrets. ARM_CLIENT_ID }}

Note that the ARM_USE_OIDC env var is necessary to be used for the AzureRM provider.

Fix #436.