FR: add support for ephemeral resources to provider

[tobiasehlert]

Is your feature request related to a problem? Please describe.
It's never good to have sensitive information stored in Terraform state and the use of the tailscale_tailnet_key resource is therefore nothing I like doing, but there are not so many other options as of today.

Describe the solution you'd like
I'd like to add support for ephemeral resources in the tailscale/tailscale provider. Ephemeral resources are Terraform resources that are temporary, have a unique lifecycle and don't get stored into the Terraform state.

Additional context
Here is an example of what I'd like to do, whose resource output I'd feed into another module/resource (like Azure Key Vault):

ephemeral "tailscale_tailnet_key" "tailnet_key" {
  ephemeral     = true
  expiry        = 900
  preauthorized = true
  tags          = ["tag:ts-node-1337"]
}

Here is more about the ephemeral resource:
https://developer.hashicorp.com/terraform/language/resources/ephemeral