Allow for immutable session variables

In MariaDB server, the session variables typically obtain their values from the global ones; also typically, the session user has the right to change the values of the session variables.

In certain cases, such a change may be highly undesired; a good example would be a regression when a previously working value for a performance-related variable now leads to a server crash; in this case, a user, often unknowingly, may bring down the whole server by merely repeating a known stanza for speeding up its queries. A real-life case is MDEV-21554.

This ticket request the implementation of a system variable (which the should be immutable at session level) and which should contain a list of other system variables which are not allowed to be modified at session level (either by any user or, at least, by users without a SUPER or similar "powerful" privilege). Such a variable will allow the MariaDB DBA to configure certain "dangerous" variables with suitable values which the regular session user will not be allowed to change.