filebeat: add kafka input tests for SASL PLAIN and SCRAM-SHA-256

[mauri870]

Proposed commit message

Our SASL integration tests for the Kafka input currently only cover SCRAM-SHA-512. Add tests for PLAIN and SCRAM-SHA-256 as well.

Checklist

• [ ] My code follows the style guidelines of this project
• [ ] I have commented my code, particularly in hard-to-understand areas
• [ ] I have made corresponding changes to the documentation
• [ ] I have made corresponding change to the default configuration files
• [ ] I have added tests that prove my fix is effective or that my feature works. Where relevant, I have used the stresstest.sh script to run them under stress conditions and race detector to verify their stability.
• [ ] I have added an entry in ./changelog/fragments using the changelog tool.

How to test this PR locally

(cd filebeat && mage docker:composeBuild && mage docker:composeUp)
go test -tags integration -v -run ^TestSASLAuthentication$ ./filebeat/input/kafka -v -count=1
./script/stresstest.sh --tags integration ./filebeat/input/kafka ^TestSASLAuthentication$ -p 1
(cd filebeat && mage docker:composeDown)

[github-actions[bot]]

:robot: GitHub comments

Just comment with:

• run docs-build : Re-trigger the docs validation. (use unformatted text in the comment!)

[mergify[bot]]

This pull request does not have a backport label. If this is a bug or security fix, could you label this PR @mauri870? 🙏. For such, you'll need to label your PR with:

• The upcoming major version of the Elastic Stack
• The upcoming minor version of the Elastic Stack (if you're not pushing a breaking change)

To fixup this pull request, you need to add the backport labels for the needed branches, such as:

• backport-8./d is the label to automatically backport to the 8./d branch. /d is the digit
• backport-active-all is the label that automatically backports to all active branches.
• backport-active-8 is the label that automatically backports to all active minor branches for the 8 major.
• backport-active-9 is the label that automatically backports to all active minor branches for the 9 major.

[elasticmachine]

Pinging @elastic/elastic-agent-data-plane (Team:Elastic-Agent-Data-Plane)