Bitswap feature request: Fixed Peer Set Session

# What

Bitswap broadcasting of want-haves and want-blocks has been a source of network spam for many operators for some time.

Some consumers of Bitswap may want to opt out of this part of bitswap -- using it purely as a data transfer protocol and doing content routing in other ways.

It would ideal to be able to start a session with a fixed set of peers, or more ideal a fixed channel of incoming peers, and never do Broadcasts or FindProviders.