This commit modifies the k8s proxy application configuration to include a
new field named `ServerURL` which, when set, modifies
the tailscale coordination server used by the proxy. This works in the same
way as the operator and the proxies it deploys.
If unset, the default coordination server is used.
Updates https://github.com/tailscale/tailscale/issues/13358
Signed-off-by: David Bond <davidsbond93@gmail.com>