[p2p/utils] Add `manager`

[patrick-ogrady]

In many p2p applications, it is a common pattern for some peer to broadcast some data to all connected peers and perform some operation once receiving t of n responses. Likewise, peers on the other end of this broadcast may want to parse (and persist) this incoming data and send back some arbitrary message (like a signature over some component of said data).

Numerous developers have told us it would be useful to expose this basic helper for building unique p2p constructions (that have interfaces different than what is exposed by consensus::{ordered_broadcast, aggregation}).

[patrick-ogrady]

The complexity here is around designing an interface that doesn't "do too much". It may be tempting to persist received messages or batch verify returned signatures but we should pass this through to the application that has instantiated this helper.