Go-ipfs must be able to fetch files via graphsync. At the moment, the graphsync interface expects a peer with which to sync and a selector to fetch. It does not support automatically finding providers, splitting requests, etc.

Proposal: Add a (repeated) --from flag to instruct IPFS to fetch a file from a specific peer. When passed, go-ipfs should:

1. Connect to the specified peers.
2. Determine which protocols these peers support.
3. Pick the best supported transport.
4. Fetch the file with this transport.

For now, go-ipfs will not even try to split the request between multiple peers when --from is specified and at least one of the specified peers supports graphsync. As we improve graphsync/bitswap, we can add request splitting logic in future PRs.