plugin: Store a plugin's featurebits in the plugin struct

We'll collect the featurebits on-demand from all currently active plugins when
needed.

lightningd/plugin.h

@@ -26,6 +26,22 @@ enum plugin_state {
 	CONFIGURED
 };
 
+/**
+ * A plugin may register any number of featurebits that should be added to
+ * various messages as part of their manifest. The following enum enumerates
+ * the possible locations the featurebits can be added to, and are used as
+ * indices into the array of featurebits in the plugin struct itself.
+ *
+ * If you edit this make sure that there is a matching entry in the
+ * `plugin_features_type_names[]` array in plugin.c.
+ */
+enum plugin_features_type {
+	PLUGIN_FEATURES_NODE,
+	PLUGIN_FEATURES_INIT,
+	PLUGIN_FEATURES_INVOICE,
+};
+#define NUM_PLUGIN_FEATURES_TYPE (PLUGIN_FEATURES_INVOICE+1)
+
 /**
  * A plugin, exposed as a stub so we can pass it as an argument.
  */
@@ -66,6 +82,11 @@ struct plugin {
 
 	/* An array of subscribed topics */
 	char **subscriptions;
+
+	/* Featurebits for various locations that the plugin
+	 * registered. Indices correspond to the `plugin_features_type`
+	 * enum. */
+	u8 *featurebits[NUM_PLUGIN_FEATURES_TYPE];
 };
 
 /**