build-task --build-arg and --secret-build-arg should support Azure Keyvault

[SteveLasker]

Rather than having to pass in static values to build-task create, i'd like to pass in a reference to a keyvault secret.

az acr build-task create \
  -n demo42quotesapi \
  --context https://github.com/demo42/quotes \
  -t demo42/quotes-api:{{.Build.ID}} \
  -f ./src/QuoteService/Dockerfile \
  --git-access-token '{{ .Source=AzureKeyVault \
                        .Vault.Name=$AKV_NAME \
                        .Vault.Secret=$GIT_TOKEN_NAME }}' \
  --build-arg REGISTRY_NAME=$REGISTRY_NAME \
  --secret-build-arg A_CONNECTIONSTRING={{ .Source=AzureKeyVault \
                        .Vault.Name=$AKV_NAME \
                        .Vault.Secret=demo42-quotes-sql-connectionstring-eastus }} \
  --registry $ACR_NAME 

[sajayantony]

KeyVault integration is supported with MSIs for tasks - Azure/azure-cli#9317