Implement index (fast) scrubbing for regular/EC volumes via `ScrubVolume()`/`ScrubEcVolume()`.
Also rearranges existing index test files for reuse across unit tests for different modules.
* Implement RPC skeleton for regular/EC volumes scrubbing.
See https://github.com/seaweedfs/seaweedfs/issues/8018 for details.
* Minor proto improvements for `ScrubVolume()`, `ScrubEcVolume()`:
- Add fields for scrubbing details in `ScrubVolumeResponse` and `ScrubEcVolumeResponse`,
instead of reporting these through RPC errors.
- Return a list of broken shards when scrubbing EC volumes, via `EcShardInfo'.
