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.
While there is more helpful information in a panic, this is confusing
to the user. With the amount of code using reflection, it seems better
to have clearer messages to help the user find the faulty section if
any.
This is a first step to accept another kind of GeoIP database (like
City). This also introduces the way we want to deprecate stuff:
transform the map structure.
