add flag for setting custom bittorrent tracker and/or peers for snapshot downloader

[taratorio]

request from @tornadocontrib on discord (discussion)

Currently we use these public trackers that are hardcoded in the code here to get bittorrent peers. We can add a flag to allow users to change that to their custom tracker. Similarly it would be nice if we have an option to specify a peer list directly in addition to that.

Potential use cases may be:

• from Alex: "i have large private infrastructure. i want to start our own internal Downloader server X (like go run ./cmd/downloader --seedbox) - and I would like all our new Erigon nodes download data from X (so traffic goes by private network instead of public - which is cheaper and faster)."
• from @tornadocontrib: "retrieving backups with validation or seeding multiple clusters of nodes ( can be useful for rpc providers )"

[dvovk]

It can't be solved by just adding a flag, we need to do much more work with the torrent lib and downloader. We've gotten webseeds download working good but still have some issues in p2p flow. I think this issue should be a part of bigger one. I think it makes sense to create "downloader" project.

[shohamc1]

@dvovk I believe the torrent client supports this natively: https://pkg.go.dev/github.com/anacrolix/torrent#Torrent.AddPeers / https://pkg.go.dev/github.com/anacrolix/torrent#Torrent.AddTrackers

[dvovk]

@shohamc1 I didn't say that it isn't supported I said that in order to make p2p work on the same level as webseeds (Mark made significant changes). At the moment p2p isn't well tested and I know that it has issues with download/upload speed. It is work which still needs to be done.

[shohamc1]

@dvovk Ah, I misunderstood you then. Of course we would need to address the p2p speed issue - I reckon this will be a smaller issue if the torrent peers are on the same network (which seems to be the primary use case for this).

Perhaps we can ban webseeds if this flag is enabled? (not sure if the library supports this)