fix: do not override SASL mechanism for user REST proxy

[Acuion]

About this change - What it does

If sasl_mechanism already defined in the config - do not override it with PLAIN.
Our Kafka installation allows SCRAM authentication only, so current code leads to failed authentication. Preferred mechanism already stated in the config, so let's use it for the user proxy.

Why this way

config["sasl_mechanism"] default value is None, so let's check for it. I'm not sure if we can remove this line completely and rely on that undefined SASL mechanism often means PLAIN. This is the most safe patch - keeping existing behavior.

[tvainika]

Looks good. This is also safe approach. Karapace configuration uses shared sasl_mechanism and some other configuration values for both REST proxy and schema registry, so it is better to fallback to PLAIN if not configured.