Add a default 'plugins' directory in the lightning dir

Automatically load all plugins from 'lightning_dir/plugins' (setting them as plugin_dir)

lightningd/plugin.h

@@ -30,6 +30,12 @@ struct plugin_opt;
 struct plugins *plugins_new(const tal_t *ctx, struct log_book *log_book,
 			    struct lightningd *ld);
 
+/**
+ * Search for `default_dir`, and if it exists add every directory it
+ * contains as a plugin dir.
+ */
+void plugins_add_default_dir(struct plugins *plugins, const char *default_dir);
+
 /**
  * Initialize the registered plugins.
  *