outputs: add Kafka output handler

[abhihashi]

Adds a new kafka output handler that publishes collected Grove logs to Apache Kafka or Confluent Platform.

• Optional dep: pip install grove[kafka]
• serialize() returns plain NDJSON; Kafka producer handles compression (lz4)
• submit() chunks by max_records_per_message=500 and max_bytes_per_message=750000
• descriptor overrides the default topic for per-operation routing
• use_identity_as_key sets Kafka message key to the connector identity
• Delivery errors and flush timeouts surface as AccessException

Files changed:

• grove/outputs/kafka.py: new output handler
• tests/test_outputs_kafka.py: 15 unit tests
• pyproject.toml: kafka entrypoint and confluent-kafka optional dependency
• docs/configuration.rst: Kafka output configuration reference

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[hashicorp-cla-app]

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