[ipfs/go-bitswap] Passing Auth to exchange.Fetcher requests

One thing I don't see covered in https://github.com/ipfs/go-bitswap/pull/543 or https://github.com/ipfs/specs/pull/270 is how we actually pass an auth token to a bitswap want from the standpoint of the library consumer.

I can see a couple mechanisms:
1. Continue the session id in context thread (https://github.com/ipfs/go-ipfs-exchange-interface/pull/9/files) and extend it for auth (i.e. auth per session)
2. Modify the exchange.Fetcher function parameters

The first sounds simpler, but is quite limited. I think we need to figure out the granularity of scope we support for passing tokens. Should our mapping be simply token per session? Token per GetBlock/GetBlocks requests? Token per peer that may send us the block? Any combination thereof?

For reference: a go-data-transfer integration will be easiest if we support per-peer, but that's also the hardest to implement :(