SPIGOT-2903: Add Structure API

By: Sander Knauff <sanderknauff@hotmail.com>

paper-api/src/main/java/org/bukkit/structure/Palette.java

@@ -0,0 +1,33 @@
+package org.bukkit.structure;
+
+import java.util.List;
+import org.bukkit.block.BlockState;
+import org.jetbrains.annotations.NotNull;
+
+/**
+ * Represent a variation of a structure.
+ *
+ * Most structures, like the ones generated with structure blocks, only have a
+ * single variant.
+ */
+public interface Palette {
+
+    /**
+     * Gets a copy of the blocks this Palette is made of.
+     *
+     * The {@link BlockState#getLocation() positions} of the returned block
+     * states are offsets relative to the structure's position that is provided
+     * once the structure is placed into the world.
+     *
+     * @return The blocks in this palette
+     */
+    @NotNull
+    List<BlockState> getBlocks();
+
+    /**
+     * Gets the number of blocks stored in this palette.
+     *
+     * @return The number of blocks in this palette
+     */
+    int getBlockCount();
+}