Refactor: move logic out of client for easier unit testing

[iliakur]

What does this PR do?

This is a big PR, so I split it into meaningful commits!
At the same time, the commits are incremental steps to the final result. So it makes sense to review them all in sequence as a story in a way.

Motivation

We need to expand the test coverage for this integration in response to support cases (and to prevent new ones).

Unit tests are better suited for this, but they work best with as little as possible mocking.

The existing code structure encouraged more extensive mocking thus reducing coverage that we could achieve with unit tests.

FUN FACT: our branch coverage by unit tests went from 66% to 92%. Without adding any new unit tests!

Review checklist (to be filled by reviewers)

• Feature or bugfix MUST have appropriate tests (unit, integration, e2e)
• Add the qa/skip-qa label if the PR doesn't need to be tested during QA.
• If you need to backport this PR to another branch, you can add the backport/<branch-name> label to the PR and it will automatically open a backport PR once this one is merged

[codecov]

Codecov Report

Attention: Patch coverage is 91.97861% with 15 lines in your changes missing coverage. Please review.

Project coverage is 87.25%. Comparing base (6c94834) to head (c26ad0a).
Report is 13 commits behind head on master.

Additional details and impacted files

Flag	Coverage Δ
activemq	?
cassandra	?
hive	?
hivemq	?
hudi	?
ignite	?
jboss_wildfly	?
kafka	?
kafka_consumer	92.77% <91.97%> (+38.51%)	⬆️
presto	?
solr	?

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