[filebeat] Add sticky partition rebalance strategy to Kafka input

[lucabelluccini]

Summary

• Adds support for the sticky partition rebalance strategy to the Filebeat Kafka input.
• Allows users to configure sticky as the rebalance.strategy option, which minimises partition movement across rebalances for improved consumer stability.

Test plan

• Unit tests pass (go test ./filebeat/input/kafka/...)
• Manually verify Kafka input with rebalance.strategy: sticky connects and consumes messages correctly
• Verify existing rebalance strategies (range, roundrobin) are unaffected

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[github-actions]

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[mergify]

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If this is a bug or security fix, could you label this PR @lucabelluccini? 🙏.
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[github-actions]

🔍 Preview links for changed docs

• docs/reference/filebeat/filebeat-input-kafka.md

[github-actions]

✅ Vale Linting Results

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ℹ️ Recent review info

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📥 Commits

Reviewing files that changed from the base of the PR and between 371e789 and 13a0db0.

📒 Files selected for processing (1)

• filebeat/input/kafka/kafka_integration_test.go

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📝 Walkthrough

Walkthrough

This PR adds support for the sticky Kafka consumer group rebalance strategy to Filebeat's Kafka input. The implementation adds a new rebalanceStrategySticky enum constant, registers "sticky" in the configuration mapping, and maps the enum value to Sarama's sticky rebalance implementation. Documentation is updated to reflect the new strategy option alongside the existing "range" (default) and "roundrobin" strategies.

🚥 Pre-merge checks | ✅ 2

✅ Passed checks (2 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

• 🛠️ Update Documentation

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[infra-vault-gh-plugin-prod]

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

[cmacknz]

@khushijain21 how much work is it to support this in the kafkaexporter too? Looks like it is supported in franz-go.

We can introduce this as long as we can support it for beats receivers going forward as well.