SPIGOT-7034: Add methods for set/get instrument in Goat Horn

By: byquanton <32410361+byquanton@users.noreply.github.com>

paper-api/src/main/java/org/bukkit/inventory/meta/MusicInstrumentMeta.java

@@ -0,0 +1,27 @@
+package org.bukkit.inventory.meta;
+
+import org.bukkit.MusicInstrument;
+import org.jetbrains.annotations.NotNull;
+import org.jetbrains.annotations.Nullable;
+
+public interface MusicInstrumentMeta extends ItemMeta {
+
+    /**
+     * Sets the goat horn's instrument.
+     *
+     * @param instrument the instrument to set
+     */
+    void setInstrument(@Nullable MusicInstrument instrument);
+
+    /**
+     * Gets the instrument of the goat horn.
+     *
+     * @return The instrument of the goat horn
+     */
+    @Nullable
+    MusicInstrument getInstrument();
+
+    @Override
+    @NotNull
+    MusicInstrumentMeta clone();
+}