MDEV-15935 Adding global/session system var redirect_url

Adding a global/session var `redirect_url' of string type. The initial
value is empty. Can be supplied in mysqld with --redirect-url or set
in --init-connect. A valid redirect_url should be of the format

{mysql,mariadb}://host[:port]

where <host> is an arbitrary string not containing colons, and <port>
is a number between 0 and 65535 inclusive.

The variable will be used by the server to notify clients that they
should connect to another server, specified by the value of the
variable, if not empty.

The notification is done by the inclusion of the variable in
session_track_system_variable.

sql/sql_class.h

@@ -883,6 +883,7 @@ typedef struct system_variables
 
   Time_zone *time_zone;
   char *session_track_system_variables;
+  char *redirect_url;
 
   /* Some wsrep variables */
   ulonglong wsrep_trx_fragment_size;