Fix compatibility with Kafka >= 3.8

[juha-aiven]

Stop using io.aiven.kafka.auth.audit.Session from Kafka that is no longer present in Kafka >= 3.8.

Add io.aiven.kafka.auth.audit.Session to replace that. In the new class, add properly named getter methods and make the fields private.

Fixes #193.

[jlprat]

LGTM Thanks @juha-aiven

[jeqo]

add properly named getter methods

nit: nothing wrong on not having get prefixed methods anymore. For instance, Java Records come with accessor methods named same as attributes

For each component in the header, two members: a public accessor method with the same name and return type as the component, and a private final field with the same type as the component;

[jlprat]

Regarding the getter setter. It all boils down to when did you get POJOs best practices imprinted in your mind.
This is a class that could definitely be replaced with a Java record as it was a case class in Scala.
But, in the end, this looks like something that can be decided project wise and chose a "policy" for POJOs and style guide for some type of stylistic questions.