[rust] KV snapshot batch scanner for full primary-key reads

[fresh-borzoni]

Search before asking

• I searched in the issues and found nothing similar.

Description

We can read a bucket's current state via limit scan, but there's no way to read a whole PK table - that needs reading RocksDB snapshot files. Requires the getKvSnapshotMetadata RPC, a remote SST downloader, and embedding RocksDB. This is the missing piece for full-table DataFusion scans over PK tables.

Willingness to contribute

• I'm willing to submit a PR!