For debugging purposes, add a new C2N endpoint returning the current
netmap. Optionally, coordination server can send a new "candidate" map
response, which the client will generate a separate netmap for.
Coordination server can later compare two netmaps, detecting unexpected
changes to the client state.
Updates tailscale/corp#32095
Signed-off-by: Anton Tolchanov <anton@tailscale.com>