Expose Per Peer Simultaneous Retrieval Limit

I noticed some SP's are interested in limiting simultaneous retrievals at once on a per peer basis.

This capability has existed in GraphSync for a while: https://github.com/ipfs/go-graphsync/blob/main/impl/graphsync.go#L136

It might be worth considering exposing this as a config, similar to the SimultaneousRetrievals config that already exists.
