Add java kafka driver backend to kafka integration tests

[rukai]

progress towards #1500

This PR splits our kafka driver abstraction up so that it supports the old CPP driver and additionally supports the java kafka driver.
So far it only implements a tiny amount of the the java driver to demonstrate the functionality while setting up the abstractions we will use and keeping the PR size manageable.

In order to use j4rs in our tests we need to include the target/debug/jassets directory in the nextest archive used for distributing builds across many CI machines.
nextest does not currently support that so I have:

• Made a temporary branch to get it working here: https://github.com/rukai/nextest/tree/archive-include-legacy
  • This is what I used in this PR just to get it over the line
• Started working on this functionality in a proper upstreamable way - Proposed design for including extra files in the archive nextest-rs/nextest#1340
  • We can swap to it in a follow up PR.

The PR is not as big as it appears, the admin function has been moved unchanged into cpp.rs so that we can reimplement it within java.rs