Add basic Datapack API

Co-authored-by: Jake Potrebic <jake.m.potrebic@gmail.com>

paper-api/src/main/java/io/papermc/paper/datapack/DatapackManager.java

@@ -0,0 +1,43 @@
+package io.papermc.paper.datapack;
+
+import java.util.Collection;
+import org.jetbrains.annotations.Unmodifiable;
+import org.jspecify.annotations.NullMarked;
+import org.jspecify.annotations.Nullable;
+
+@NullMarked
+public interface DatapackManager {
+
+    /**
+     * Triggers a refresh of the available and selected datapacks. This
+     * can find new datapacks, remove old ones, and update the metadata for
+     * existing datapacks. Some of these changes will only take effect
+     * after the next {@link org.bukkit.Server#reloadData()} or {@code /minecraft:reload}.
+     */
+    void refreshPacks();
+
+    /**
+     * Gets a datapack by name. May require calling {@link #refreshPacks()} before
+     * to get the latest pack information.
+     *
+     * @param name the name/id of the datapack
+     * @return the datapack, or null if not found
+     */
+    @Nullable Datapack getPack(String name);
+
+    /**
+     * Gets the available datapacks. May require calling {@link #refreshPacks()} before
+     * to get the latest pack information.
+     *
+     * @return all the packs known to the server
+     */
+    @Unmodifiable Collection<Datapack> getPacks();
+
+    /**
+     * Gets the enabled datapacks. May require calling {@link #refreshPacks()} before
+     * to get the latest pack information.
+     *
+     * @return all the packs which are currently enabled
+     */
+    @Unmodifiable Collection<Datapack> getEnabledPacks();
+}