Add a narrow LocalAPI accessor and matching client/LocalBackend method
to look up a single peer's current full [tailcfg.Node] by NodeID, in
O(1) time on the daemon side, without fetching the entire netmap.
Useful for callers that need the latest state of a single peer (e.g.
in response to a peer-mutation event on the IPN bus) without paying
for a full netmap fetch.
Updates #12542
Change-Id: I1cb2d350e6ad846a5dabc1f5368dfc8121387f7c
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
89a78dc9b7