RFE: ACR Managed Identity support in the GitHub Action

Only service_principal & service_principal_password can be provided in the GitHub Action.
When ACR is set with Managed Identity, how to provide the identity to use to connect to ACR ?

see also [https://github.com/Azure/acr-build/issues/10](https://github.com/Azure/acr-build/issues/10)

```sh
env:
AZURE_CONTAINER_REGISTRY: acrpetcliaca
REGISTRY_URL: acrpetcliaca.azurecr.io # set this to the URL of your registry
REPOSITORY: petclinic # set this to your ACR repository
PROJECT_NAME: petclinic # set this to your project's name
KV_NAME: kv-petcliaca442

RG_KV: rg-iac-kv777
RG_APP: rg-iac-aca-petclinic-mic-srv

# ==== APPS ====
PRJ_PREFIX: aca-spring-petclinic

API_GATEWAY: api-gateway
ADMIN_SERVER: admin-server
CUSTOMERS_SERVICE: customers-service
VETS_SERVICE: vets-service
VISITS_SERVICE: visits-service

CONFIG_SERVER: config-server
DISCOVERY_SERVER: discovery-server

# https://github.com/Azure/actions-workflow-samples/blob/master/assets/create-secrets-for-GitHub-workflows.md#consume-secrets-in-your-workflow
# https://docs.github.com/en/actions/using-workflows/workflow-syntax-for-github-actions#example-using-secrets
# ==== Secrets ====
credentials: ${{ secrets.AZURE_CREDENTIALS }}
AZURE_TENANT_ID: ${{ secrets.AZURE_TENANT_ID }}
SPN_ID: ${{ secrets.SPN_ID }}
SPN_PWD: ${{ secrets.SPN_PWD }}

- name: admin-server ACR build
id: admin-server-acr-build
uses: azure/acr-build@v1
with:
service_principal: ${{ env.SPN_ID }} # The Service Principal credentials : must be stored in KV
service_principal_password: ${{ env.SPN_PWD }} # The Service Principal credentials: must be stored in KV
tenant: ${{ env.AZURE_TENANT_ID }} # The ACR tenant
registry: ${{ env.AZURE_CONTAINER_REGISTRY }} # The ACR name
repository: ${{ env.REPOSITORY }} # The repository on the ACR
image: ${{ env.ADMIN_SERVER }} # Docker image name. Default tag: The action will default to use the first 8 characters of the commit SHA if no tag is specified. Tagged with GitHub commit ID (SHA), ex: 'mcr.microsoft.com/azuredocs/containerapps-helloworld:latest'
# git_access_token: ${{ secrets.git_access_token }} # The Github access token for private repositories
folder: ${{ github.workspace }} # Build context for Docker agent
dockerfile: ./docker/petclinic-admin-server/Dockerfile # Path to the Dockerfile relative to `folder`
branch: main # Branch to root the context off of
```