Add Hopper block state and inventory type. Adds BUKKIT-3749

By: Michael Limiero <mike5713@gmail.com>

paper-api/src/main/java/org/bukkit/block/Hopper.java

@@ -0,0 +1,8 @@
+package org.bukkit.block;
+
+/**
+ * Represents a hopper.
+ */
+public interface Hopper extends BlockState, ContainerBlock {
+
+}