Kafka supports SASL authentication

[rukai]

These should be addressed in separate PRs.

## Issue 1 - race condition

To move the SASL work forward we've left a race condition in #1513 (comment)
This needs to be fixed before we start using SASL.

Issue 2 - simpler config

I dont see any fundamental reason why we need a config to enable/disable SASL in shotover.
Before we make our next release we should change our SASL support to not rely on a config forcing it on/off.