When using `mapstructure:",squash"`, most structure-specific hook don't
dive into the structure as they are provided with the parent structure.
Add an helper to make them work on the embedded structure as well and
use it for the generic "deprecated fields" hook, but also for the hook
for the common Kafka configuration.
This is a bit brittle. There are other use cases, but they may not need
this change.
76151bea66