Connection redirection: add redirect_url parameter

MaxScale supports a limited form of connection redirection in the form of configurable metadata but now with MDEV-15935 complete, MaxScale should handle it better. To behave similarly to the MariaDB server, MaxScale should have a redirect_url parameter that, once set, is sent to all existing and new clients.

Feature description in MariaDB: https://mariadb.com/kb/en/connection-redirection-mechanism-in-the-mariadb-clientserver-protocol/

Implementation

There's two ways to implement this: as a global parameter, as a listener parameter or as both a global and a listener parameter. Having a global parameter is the least flexible method as it forces all sessions in MaxScale to redirect but it is the most convenient when used to shut down a MaxScale. If implemented as a listener parameter, there's additional versatility in how it can be used (e.g. redirect from one listener in MaxScale to another) but the use-case of shutting down a MaxScale requires potentially several calls to maxctrl. Implementing both a global parameter and a listener parameter would have the best parts of both of them but it may result in somewhat confusing situations: if a global redirect_url and a listener redirect_url are defined, which one should be used? The simplest approach would be to initially implement the global option and see if use-cases crop up where separate listener parameters are needed.