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/InventoryTransactionPacket.php

@@ -37,7 +37,7 @@ use pocketmine\network\mcpe\protocol\types\UseItemTransactionData;
 /**
  * This packet effectively crams multiple packets into one.
  */
-class InventoryTransactionPacket extends DataPacket{
+class InventoryTransactionPacket extends DataPacket implements ClientboundPacket, ServerboundPacket{
 	public const NETWORK_ID = ProtocolInfo::INVENTORY_TRANSACTION_PACKET;
 
 	public const TYPE_NORMAL = 0;