cmd/k8s-operator: add a kubernetes operator.

This was initially developed in a separate repo, but for build/release
reasons and because go module management limits the damage of importing
k8s things now, moving it into this repo.

At time of commit, the operator enables exposing services over tailscale,
with the 'tailscale' loadBalancerClass. It also currently requires an
unreleased feature to access the Tailscale API, so is not usable yet.

Updates tailscale#502.

Signed-off-by: David Anderson <[email protected]>

Author: danderson