service: Support validating templated files

roles/service/meta/argument_specs.yaml

@@ -215,6 +215,14 @@ argument_specs:
             elements: str
             required: false
             default: []
+          template_validate_command:
+            description: >-
+              Command used to verify templated file validity.
+              Will be run in a temporary container using the same container image as the main service.
+              File will be available in the same place as in the service container (destination).
+              Only applicable if mount type is template
+            type: str
+            required: false
       service_container_devices:
         description: List of devices to be added inside the service main container.
         type: list