start addon API

common/api/src/main/java/com/dfsek/terra/api/addon/Addon.java

@@ -0,0 +1,4 @@
+package com.dfsek.terra.api.addon;
+
+public interface Addon {
+}

common/api/src/main/java/com/dfsek/terra/api/addon/AddonLoader.java

@@ -0,0 +1,13 @@
+package com.dfsek.terra.api.addon;
+
+import com.dfsek.terra.api.TerraPlugin;
+import com.dfsek.terra.api.registry.CheckedRegistry;
+
+
+public interface AddonLoader {
+    /**
+     * Load all addons.
+     * @param main TerraPlugin instance.
+     */
+    void load(TerraPlugin main, CheckedRegistry<Addon> addons);
+}