peerstore: add new `GetPeersForProtocol` method to `Protobook`

[synzhu]

The proposal is to add a new GetPeersForProtocol method to Protobook interface and implementations.

Currently, the protobook only maps from peers to protocols, and not the other way around. It's possible to simply iterate over all peers and filter them out by calling SupportsProtocols, but this could potentially become inefficient if we need to do this frequently and we have a large number of peers in the peerstore.

Alternatively, it is possible to listen for events (e.g EvtPeerProtocolsUpdated) and build the reverse mapping ourselves, but this is not 100% accurate as for example atm there is no event for when a protocol is removed. In fact, currently the Protobook data never gets purged at all, which is another issue entirely.

cc: @vyzo @Stebalien @marten-seemann

[Stebalien]

This sounds like a reasonable addition.

[marten-seemann]

The implementation will need to take care to not create any (additional) mutex contention.

We use this trick to reduce it for peer IDs and we might need something similar for protocols: https://github.com/libp2p/go-libp2p-peerstore/blob/a1f426f3d046825d53b6d775edef4f854f976166/pstoremem/protobook.go#L11-L20