Add unsupported config option and internal API to simplify remote item matching

This is important for 1.21.5 servers/clients and non-Vanilla clients that may not be able to match 1.21.5 data hashes anymore

paper-server/patches/sources/net/minecraft/server/level/ServerPlayer.java.patch

@@ -10,7 +10,7 @@
      @Nullable
      private Vec3 startingToFallPosition;
      @Nullable
-@@ -258,6 +_,13 @@
+@@ -258,6 +_,20 @@
              }
          }
  
@@ -20,6 +20,13 @@
 +            ServerPlayer.this.connection.send(new ClientboundContainerSetSlotPacket(ServerPlayer.this.inventoryMenu.containerId, ServerPlayer.this.inventoryMenu.incrementStateId(), net.minecraft.world.inventory.InventoryMenu.SHIELD_SLOT, ServerPlayer.this.inventoryMenu.getSlot(net.minecraft.world.inventory.InventoryMenu.SHIELD_SLOT).getItem().copy()));
 +        }
 +        // Paper end - Sync offhand slot in menus
++
++        // Paper start - add flag to simplify remote matching logic
++        @Override
++        public ServerPlayer player() {
++            return ServerPlayer.this;
++        }
++        // Paper end - add flag to simplify remote matching logic
 +
          @Override
          public void sendSlotChange(AbstractContainerMenu container, int slot, ItemStack itemStack) {