nixos/k3s: add initial k3s service

* nixos/k3s: simplify config expression

* nixos/k3s: add config assertions and trim unneeded bits

* nixos/k3s: add a test that k3s works; minor module improvements

This is a single-node test. Eventually we should also have a multi-node
test to verify the agent bit works, but that one's more involved.

* nixos/k3s: add option description

* nixos/k3s: add defaults for token/serveraddr

Now that the assertion enforces their presence, we dont' need to use the typesystem for it.

* nixos/k3s: remove unneeded sudo in test

* nixos/k3s: add to test list