Add a checksum for exported and uploaded calibnet snapshots

[LesnyRumcajs]

Issue summary

While exporting the snapshots, we should create a checksum file (optimally it should be done while exporting, i.e. in a loop with a hasher and not manually afterwards).

Those checksum files should have the same name as the car file, but with sha256sum extension, along with how it is in Filecoin mainnet snapshots (see implementation here).

Afterwards, the snapshot fetch code should be updated to validate downloaded snapshots checksum (for calibnet, for mainnet it is already implemented).

Other information and links

See issues and related PRs: https://github.com/ChainSafe/forest/issues/1846 and https://github.com/ChainSafe/forest/issues/1694