Implement adding velocity to player from Lua

The intended usecase is knockback, but there's potential for more.

src/network/networkprotocol.h

@@ -194,6 +194,7 @@ with this program; if not, write to the Free Software Foundation, Inc.,
 		New network float format
 		ContentFeatures version 13
 		Add full Euler rotations instead of just yaw
+		Add TOCLIENT_PLAYER_SPEED
 */
 
 #define LATEST_PROTOCOL_VERSION 37
@@ -295,6 +296,11 @@ enum ToClientCommand
 		u32 CSMRestrictionFlags byteflag
 	 */
 
+	TOCLIENT_PLAYER_SPEED = 0x2B,
+	/*
+		v3f added_vel
+	 */
+
 	// (oops, there is some gap here)
 
 	TOCLIENT_CHAT_MESSAGE = 0x2F,