[WIP] Update kafka-exporter module to follow style guide

[Copilot]

• Analyze current kafka-exporter code against style guide requirements
• Update KafkaSpanExporter to follow style guide (add final, requireNonNull, @nullable)
• Update KafkaSpanExporterBuilder to follow style guide (add final, static imports, requireNonNull)
• Update SpanDataSerializer to follow style guide (add final, requireNonNull)
• Update SpanDataDeserializer to follow style guide (add final)
• Run tests and spotless to ensure changes are correct
• Verify minimal changes and functionality preservation

Making minimal changes to update kafka-exporter module to follow docs/style-guide.md where it does not already.

Key changes planned:

1. Add final keyword to public classes per style guide
2. Replace validation using Objects.isNull() with requireNonNull()
3. Add appropriate static imports
4. Add @Nullable annotations where parameters/fields can be null
5. Ensure fields are final where possible

Fixes #266.

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