imap: Add client_enabled_features()

This simplifies the following commits.

src/imap/imap-client.c

@@ -1495,6 +1495,21 @@ enum mailbox_feature client_enabled_mailbox_features(struct client *client)
 	return client->enabled_features;
 }
 
+const char *const *client_enabled_features(struct client *client)
+{
+	static const char *condstore_str = "CONDSTORE";
+	static const char *qresync_str = "QRESYNC";
+	ARRAY_TYPE(const_string) features;
+	t_array_init(&features, 8);
+
+	if ((client->enabled_features & imap_feature_condstore) != 0)
+		array_append(&features, &condstore_str, 1);
+	if ((client->enabled_features & imap_feature_qresync) != 0)
+		array_append(&features, &qresync_str, 1);
+	array_append_zero(&features);
+	return array_idx(&features, 0);
+}
+
 struct imap_search_update *
 client_search_update_lookup(struct client *client, const char *tag,
 			    unsigned int *idx_r)

src/imap/imap-client.h

@@ -310,6 +310,8 @@ void client_enable(struct client *client, unsigned int feature_idx);
 bool client_has_enabled(struct client *client, unsigned int feature_idx);
 /* Returns mailbox features that are currently enabled. */
 enum mailbox_feature client_enabled_mailbox_features(struct client *client);
+/* Returns all enabled features as strings. */
+const char *const *client_enabled_features(struct client *client);
 
 /* Send client processing to imap-idle process. If successful, returns TRUE
    and destroys the client. */