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

request from @tornadocontrib on discord ([discussion](https://discord.com/channels/687972960811745322/983710221308416010/1329775324803895336))

Currently we use these public trackers that are hardcoded in the code [here](https://github.com/erigontech/erigon/blob/main/erigon-lib/downloader/util.go#L52-L58) 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 )"