wgengine/netlog: embed node information in network flow logs (#17668)

This rewrites the netlog package to support embedding node information in network flow logs.
Some bit of complexity comes in trying to pre-compute the expected size of the log message
after JSON serialization to ensure that we can respect maximum body limits in log uploading.

We also fix a bug in tstun, where we were recording the IP address after SNAT,
which was resulting in non-sensible connection flows being logged.

Updates tailscale/corp#33352

Signed-off-by: Joe Tsai <joetsai@digital-static.net>

wgengine/netlog/netlog_omit.go

@@ -7,7 +7,8 @@ package netlog
 
 type Logger struct{}
 
-func (*Logger) Startup(...any) error { return nil }
-func (*Logger) Running() bool        { return false }
-func (*Logger) Shutdown(any) error   { return nil }
-func (*Logger) ReconfigRoutes(any)   {}
+func (*Logger) Startup(...any) error   { return nil }
+func (*Logger) Running() bool          { return false }
+func (*Logger) Shutdown(any) error     { return nil }
+func (*Logger) ReconfigNetworkMap(any) {}
+func (*Logger) ReconfigRoutes(any)     {}