peerstore: add a `PeersSupporting(proto string, seenWithin time.Duration) <-chan peer.ID` capability

Nobody has claimed this yet.

The peerstore doesn't support obtaining an enumeration of peers known to support a given protocol. Service like AutoNAT thus [keep peers relevant in memory throughout the lifetime of the process](https://github.com/libp2p/go-libp2p-autonat/blob/master/autonat.go#L55), which in the long run leaks memory.

Adding a capability like this with our current KV model is not trivial. We might need to migrate to SQLite, or another DB richer than simple KV stores.

Start by reading the peerstore implementation and its current key-value storage model, then inspect the linked AutoNAT usage to understand the memory-retention problem. Determine whether the existing storage can support enumerating peers by protocol and recency, or whether the proposed SQLite or richer database migration is required. Done means exposing the requested capability without retaining all relevant peers for the process lifetime.

Written by the indexing model from the issue text.