Add support for minimum Cloud Run container instances (`min_scale`)

Add min_scale variable to control the minimum number of Cloud Run container instances. Similar to max_scale, the variable should be used to set the autoscaling.knative.dev/minScale annotation key on the google_cloud_run_service resource.

See https://gitlab.developers.cam.ac.uk/uis/devops/infra/terraform/gcp-cloud-run-app/-/blob/master/main.tf#L37