[rust] BatchScanner: whole-table scan via create_batch_scanner()

[fresh-borzoni]

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Description

Add the no-arg create_batch_scanner() that scans every bucket (and partition) of a table, not just one.

Internally it fans out per-bucket LimitBatchScanners and applies a global limit across them (Java's CompositeBatchScanner).
Reserve the bare name for this, the per-bucket one is create_bucket_batch_scanner.

Willingness to contribute

• I'm willing to submit a PR!