Also perform minor cleanups on the ctxkey package itself.
Provide guidance on when to use ctxkey.Key[T] over ctxkey.New.
Also, allow for interface kinds because the value wrapping trick
also happens to fix edge cases with interfaces in Go.
Updates #cleanup
Signed-off-by: Joe Tsai <joetsai@digital-static.net>
Add a standalone server for STUN that can be hosted independently of the
derper, and factor that back into the derper.
Fixes#8434Closes#8435Closes#10745
Signed-off-by: James Tucker <james@tailscale.com>