Support "pg_promote()" function (PostgreSQL 12 and later)

This is an experimental feature.

dbutils.h

@@ -424,7 +424,7 @@ ExtensionStatus get_repmgr_extension_status(PGconn *conn);
 /* node management functions */
 void		checkpoint(PGconn *conn);
 bool		vacuum_table(PGconn *conn, const char *table);
-
+bool		promote_standby(PGconn *conn, bool wait, int wait_seconds);
 
 /* node record functions */
 t_server_type parse_node_type(const char *type);