Added Key Vault access policies for App Service identities in az-isolated-service-single-region template

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What is the current behavior?

The az-isolated-service-single-region template creates all app services in Azure with system assigned identities; however, the Key Vault does not have an access policy for those identities to allow them Get/List permissions to the resource objects.

Issue Number: #282

What is the new behavior?

[x] App services defined within the unauthn_deployment_targets will also have a key vault access policy created providing them with GET/LIST permissions
[x] App services defined within the authn_deployment_targets will also have a key vault access policy created providing them with GET/LIST permissions

Does this introduce a breaking change?

[NO]

While the behavior will be different, it should only add the permission to any existing deployments.

Any relevant logs, error output, etc?

N/A

Other information

N/A

microsoft / cobalt

Added Key Vault access policies for App Service identities in az-isolated-service-single-region template #285