Expose log_data blobs in WriteBatchIterator and WriteBatchIteratorCf

[lucasvuillier]

Summary

• add optional log_data(&mut self, blob: &[u8]) method (default: no-op) to both WriteBatchIterator and WriteBatchIteratorCf
• add rust_rocksdb_writebatch_iterate and rust_rocksdb_writebatch_iterate_cf to c-api-extensions: thin C++ wrappers over WriteBatch::Handler that expose the LogData callback
  the upstream rocksdb_writebatch_iterate / rocksdb_writebatch_iterate_cf don't forward
• wire up the corresponding FFI callbacks in iterate() and iterate_cf() so blobs written via put_log_data are forwarded to the Rust layer instead of being silently dropped
• existing implementations of both traits require no changes (provided method with default no-op)

Testing

• new test_write_batch_iterate_log_data — verifies WriteBatchIterator::log_data is called for each put_log_data entry in order
• new test_write_batch_cf_iterate_log_data — same for WriteBatchIteratorCf
• new writebatch_log_data phase in librocksdb-sys/tests/ffi.rs — calls rust_rocksdb_writebatch_iterate with a CheckLogData callback that asserts content and call order