SPIGOT-2706: Implement support for Lock NBT Tag

Containers may now implement the Lockable interface.

By: Senmori <thesenmori@gmail.com>

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

@@ -0,0 +1,31 @@
+package org.bukkit.block;
+
+/**
+ * Represents a block (usually a container) that may be locked. When a lock is
+ * active an item with a name corresponding to the key will be required to open
+ * this block.
+ */
+public interface Lockable {
+
+    /**
+     * Checks if the container has a valid (non empty) key.
+     *
+     * @return true if the key is valid.
+     */
+    boolean isLocked();
+
+    /**
+     * Gets the key needed to access the container.
+     *
+     * @return the key needed.
+     */
+    String getLock();
+
+    /**
+     * Sets the key required to access this container. Set to null (or empty
+     * string) to remove key.
+     *
+     * @param key the key required to access the container.
+     */
+    void setLock(String key);
+}