#1067: Add method to get Advancement requirements

By: Nothixal <nothixal@gmail.com>

paper-api/src/main/java/org/bukkit/advancement/AdvancementRequirement.java

@@ -0,0 +1,23 @@
+package org.bukkit.advancement;
+
+import java.util.List;
+import org.jetbrains.annotations.NotNull;
+
+public interface AdvancementRequirement {
+
+    /**
+     * Get all required criteria.
+     *
+     * @return the list of required criteria for this requirement.
+     */
+    @NotNull
+    List<String> getRequiredCriteria();
+
+    /**
+     * Check if the requirement is strict.
+     *
+     * @return true if requirement list contains one criteria, false if
+     * multiple.
+     */
+    boolean isStrict();
+}