This adds a new NodeAgentClient type that can be used to
invoke the LocalAPI using the LocalClient instead of
handcrafted URLs. However, there are certain cases where
it does make sense for the node agent to provide more
functionality than whats possible with just the LocalClient,
as such it also exposes a http.Client to make requests directly.
Signed-off-by: Maisem Ali <maisem@tailscale.com>
