FM-362: Save snapshot chunks

[aakoshh]

Closes consensus-shipyard/ipc#179

• Split the snapshot crate into multiple submodules to manage code size
• Change App::offer_snapshot to put into the state a new SnapshotDownload record which shows which manifest we are downloading to what temporary directory
• SnapshotDownload cleans up the temporary directory when the last clone of it is dropped, e.g. if CometBFT restarts with a new snapshot
• Add App::apply_snapshot_chunk to ingest new chunks of a snapshot, saving them to the temporary directory and validating the checksum when they are all present

Not done in this PR:

• Tests are deferred to End-to-end test for snapshots ipc#178
• Any locking - the docs say CometBFT calls apply_snapshot_chunk sequentially, but I'm not sure if it waits before passing the next chunk, or is it similar to end_block which was called while deliver_tx was still running. We'll see.
• Here we just save all the snapshot parts and verify the checksum at the end; the import of the snapshot is deferred to Commit snapshots to the blockstore ipc#176
• Not trying to load partial CAR files into memory to check that the CIDs have been referenced by something seen earlier; this is deferred to Validate snapshots during/after loading ipc#177

[aakoshh]

I'm not sure yet if we should import the snapshot here, or make another call from the App. In any case, the App will have to write to its app state history, so we'll have to return the full SnapshotManifest so it can construct the necessary record.

The benefit of returning from here would be just a bit of extra visibility in the logs, showing explicitly when we start the import.

[aakoshh]

This change was done in 6e3c587

[adlrocha]

I had a few questions on how you import and if you already persist this snapshot so it is available to offer it to other peers, but I see this is addressed in the follow-up PR. Will ask my questions there if they are still not solved. 🙏

[adlrocha]

I had a few questions on how you import and if you already persist this snapshot so it is available to offer it to other peers, but I see this is addressed in the follow-up PR. Will ask my questions there if they are still not solved. 🙏