Introduce an optional extension point for Endpoint that enables a path
for WireGuard to inform an integration about the peer public key that is
associated with an Endpoint.
The API is expected to return either the same or a new Endpoint in
response to this function. A future version of this patch could
potentially remove the returned Endpoint, but would require larger
integrator changes downstream.
This adds a small per-packet cost that could later be removed with a
larger refactor of the wireguard-go interface and Tailscale magicsock
code, as well as introducing a generic bound for Endpoint in a device &
bind instance.
Updates tailscale/corp#20732
2f5d148bcf