Is your feature request related to a problem? Please describe.
I'm managing a Tailscale setup using Terraform. The IaC setup hasn't been well maintained and as a result the tailnet key has fallen out-of-state.

Describe the solution you'd like
I would like to be able to import our existing key to the tailscale_tailnet_key resource.

Additional context
It appears that this and two other resources were left out of an earlier effort to add import functionality for all resources: #441 (comment)

I've never worked directly on Terraform providers before, but given some time I should be able to cut a PR for the tailnet_key resource.