Stable state ID for list/query-type data source

[neinkeinkaffee]

The provider currently generates a new UUID for tailscale_devices and tailscale_users and sets is as resource ID in state. This means that every plan/apply results in a diff. We should look into setting a stable ID.

Other providers variously set the ID of list/query-type resources

• not at all (resulting in a null ID in state), e.g. the coffees data source in the HashiCups provider and the ECR images data source in the AWS provider
• to a stable property of the provider, e.g. the artifact_registry_docker_images data source of the Google Cloud provider
• to a hash of a combination of the data source attributes e.g. the ec2_ami_ids data source in the AWS provider

We could pick one of those options.