Increase kafka client test coverage

[cescoffier]

Fixes #8025

add a few tests related to the Kafka client.

Unfortunately, Avro does not work in native mode (fields written using unsafe).
I will create an issue to track this.

[cescoffier]

Forgot to disable the test if docker is not enabled. Will fix that.

[gunnarmorling]

LGTM, a few minor comments inline.

[gsmet]

@cescoffier can you check Gunnar's comments?

[cescoffier]

@gunnarmorling I need to look at the Apicurio version of the Avro serde. So far the need was the "original" Avro. I can see an opportunity here if it works flawlessly in native.
I have created #8541.

[gsmet]

I really don't like the fact that we have versions in the pom files. They should either be in the BOM if it makes sense or in the build-parent.

[cescoffier]

As these artifacts are not available from Maven Central it's tricky to add them to the dependency management from the parent. That's why I put the version there on purpose.

Someone not aware of this would add the dependency that won't get resolved.
Note that we should NOT declare the repository in the parent either.

[gsmet]

I rebased and force pushed as it was an old PR. I'm a bit worried about the external repository. We'll see if it causes problems and reevaluate.