net/wsconn: accept a remote addr string and plumb it through

This makes wsconn.Conns somewhat present reasonably when they are
the client of an http.Request, rather than just put a placeholder
in that field.

Updates tailscale/corp#13777

Signed-off-by: David Anderson <danderson@tailscale.com>

derp/derphttp/websocket.go

@@ -27,6 +27,6 @@ func dialWebsocket(ctx context.Context, urlStr string) (net.Conn, error) {
 		return nil, err
 	}
 	log.Printf("websocket: connected to %v", urlStr)
-	netConn := wsconn.NetConn(context.Background(), c, websocket.MessageBinary)
+	netConn := wsconn.NetConn(context.Background(), c, websocket.MessageBinary, urlStr)
 	return netConn, nil
 }