New workload: eventdata (Apache web access logs)

Port the OSB `eventdata` workload. ~20M documents / ~15 GB of Apache access logs. High-throughput append-only indexing benchmark with realistic log-shaped documents — useful for measuring update handler and commit performance.

> ⚠️ **Prerequisite:** Verify dataset licence is ASF-compatible before starting.

## Tasks
- Confirm dataset licence
- Convert OSB workload using `solr-orbit convert-workload`
- Define operations: bulk indexing, basic log search, status-code faceting
- Add 1k sample corpus for test-mode
- Check whether any operations belong in `common_operations/` rather than this workload

**Depends on:** #3 (ASF dataset hosting must be resolved before corpus files can be finalised)

## References
- OSB workload: https://github.com/opensearch-project/opensearch-benchmark-workloads/tree/main/eventdata
- Creating workloads: https://github.com/apache/solr-orbit/blob/main/CREATE_WORKLOAD_GUIDE.md