Add command line option to load extra plugin jars not in the plugins folder

ex: java -jar paperclip.jar nogui -add-plugin=/path/to/plugin.jar -add-plugin=/path/to/another/plugin_jar.jar

paper-api/src/main/java/org/bukkit/Bukkit.java

@@ -83,6 +83,20 @@ public final class Bukkit {
         return server;
     }
 
+    /**
+     * Returns the de facto plugins directory, generally used for storing plugin jars to be loaded,
+     * as well as their {@link org.bukkit.plugin.Plugin#getDataFolder() data folders}.
+     *
+     * <p>Plugins should use {@link org.bukkit.plugin.Plugin#getDataFolder()} rather than traversing this
+     * directory manually when determining the location in which to store their data and configuration files.</p>
+     *
+     * @return plugins directory
+     */
+    @NotNull
+    public static File getPluginsFolder() {
+        return server.getPluginsFolder();
+    }
+
     /**
      * Attempts to set the {@link Server} singleton.
      * <p>