Create Kafka Admin Client outside of the Parallel scheduler thread pool

[tnewman-at-gm]

• Breaking change? (if so, please describe the impact and migration path for existing application instances)

What changes did you make? (Give an overview)
I identified a bug where Event Hub with Kafka protocol and Entra authentication does not work because.block() is being called in a parallel scheduler thread, which throws an exception. Creating the Admin client using Mono.supplier() runs on the parallel scheduler. Creating the admin client using a CompletableFuture eliminates this problem because CompletableFutures are run on different threads.

Here is the bug:

Here is the result after the fix:

Is there anything you'd like reviewers to focus on?

How Has This Been Tested? (put an "x" (case-sensitive!) next to an item)

• No need to
• Manually (please, describe, if necessary) - After making the change, I started Kafka UI and connected to an Azure Event Hub Cluster using Kafka protocol and Entra authentication. I can now list topics.
• Unit checks
• Integration checks
• Covered by existing automation

Checklist (put an "x" (case-sensitive!) next to all the items, otherwise the build will fail)

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation (e.g. ENVIRONMENT VARIABLES)
• My changes generate no new warnings (e.g. Sonar is happy)
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged

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[yeikel]

@Haarolean Can you take a look?