Support SASL/SCRAM authentication in the Kafka input.

[faec]

What does this PR do?

This is a clone of #30791 by @lpflpf, which enables SASL/SCRAM authentication in the Kafka input. This PR is identical to the previous one except it adds:

• an integration test exercising the new flags
• a reference configuration for the Kafka input (which was apparently missing from the documentation)

(...since the original PR was idle for a while and I don't have permission to edit that branch directly.)

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
• I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

How to test this PR locally

This test uses the Kafka test container. To try it without running the full integration test suite, start the container by itself:

docker build -t kafka testing/environments/docker/kafka
docker run -p 127.0.0.1:9092:9092/tcp -p 127.0.0.1:9093:9093/tcp --rm --name kafkatest -d kafka

and remove the //go:build integration guards on the test file before running go test.

Closes #31086.

[elasticmachine]

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

[mergify]

This pull request is now in conflicts. Could you fix it? 🙏
To fixup this pull request, you can check out it locally. See documentation: https://help.github.com/articles/checking-out-pull-requests-locally/

git fetch upstream
git checkout -b kafka-input-sasl upstream/kafka-input-sasl
git merge upstream/main
git push upstream kafka-input-sasl

[elasticmachine]

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• Start Time: 2022-04-05T19:40:07.435+0000

• Duration: 70 min 20 sec

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Test	Results
Failed	0
Passed	6192
Skipped	728
Total	6920

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