Add a variable and flag to check the configuration without actually starting the server

It's possible to misspell a variable name in any of the config files.
It would be useful to have a flag or setting like MaxScale's --config-check to validate the configuration before actually starting the server on it.
There are some scenarios when this will be very useful for example as in MDEV-31517 case.

Adding to that, I'm also of the advice to have such setting enabled by default that is, do not attempt to start if any variable is misspelled.

I imagine that presents a technical challenge for several reasons.