Implements the controller for the new ProxyGroup CRD, designed for
running proxies in a high availability configuration. Each proxy gets
its own config and state Secret, and its own tailscale node ID.
We are currently mounting all of the config secrets into the container,
but will stop mounting them and instead read them directly from the kube
API once #13578 is implemented.
Updates #13406
Signed-off-by: Tom Proctor <tomhjp@users.noreply.github.com>
