Add restrictions on packet send directions with interfaces

This prevents plugins sending wrong packets at the compiler level (or would, if we had a compiler). It's more robust than a getter for client/server and throwing an exception since a static analysis tool can detect faults created by sending wrong packets from the server. This is also used to deny service to dodgy clients which send wrong packets to the server to attack it.

src/pocketmine/network/mcpe/protocol/LoginPacket.php

@@ -43,7 +43,7 @@ use function is_array;
 use function json_decode;
 use function json_last_error_msg;
 
-class LoginPacket extends DataPacket{
+class LoginPacket extends DataPacket implements ServerboundPacket{
 	public const NETWORK_ID = ProtocolInfo::LOGIN_PACKET;
 
 	public const EDITION_POCKET = 0;