(Backport) Introduce EntitySpawnExtension (#160)

* Introduce EntitySpawnExtension

* Add to mixins

* Update common/src/main/java/dev/architectury/extensions/network/EntitySpawnExtension.java

* Update common/src/main/java/dev/architectury/networking/NetworkManager.java

Co-authored-by: Juuxel <6596629+Juuxel@users.noreply.github.com>

forge/src/main/java/dev/architectury/mixin/forge/MixinEntitySpawnExtension.java

@@ -0,0 +1,38 @@
+/*
+ * This file is part of architectury.
+ * Copyright (C) 2020, 2021 architectury
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 3 of the License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public License
+ * along with this program; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA.
+ */
+
+package dev.architectury.mixin.forge;
+
+import dev.architectury.extensions.network.EntitySpawnExtension;
+import net.minecraft.network.FriendlyByteBuf;
+import net.minecraftforge.entity.IEntityAdditionalSpawnData;
+import org.spongepowered.asm.mixin.Mixin;
+
+@Mixin(EntitySpawnExtension.class)
+public interface MixinEntitySpawnExtension extends IEntityAdditionalSpawnData {
+    @Override
+    default void writeSpawnData(FriendlyByteBuf buf) {
+        ((EntitySpawnExtension) this).saveAdditionalSpawnData(buf);
+    }
+    
+    @Override
+    default void readSpawnData(FriendlyByteBuf buf) {
+        ((EntitySpawnExtension) this).loadAdditionalSpawnData(buf);
+    }
+}

forge/src/main/resources/architectury.mixins.json

@@ -11,6 +11,7 @@
     "MixinBlockEntity",
     "MixinBlockEntityExtension",
     "MixinChunkSerializer",
+    "MixinEntitySpawnExtension",
     "MixinFallingBlockEntity",
     "MixinItemExtension",
     "MixinRegistryEntry",