discovery: Add Peers method

[Wondertan]

We decided to rely on existing discovery primitives. It is a single-purpose mechanism providing the API for
FN/BNs to advertise themselves, s.t. other LN/FN/BNs can discover and ensure N connections to them. However, the
current API does not let us know our discovered FN/BN peers' connections. A listing of discovered peers is required for
ShrEx/EDS and ShrEx/ND, as their clients do not discover servers automatically and expect them to be provided.

Subsequently, we extend discovery.Discovery with the following method:

// Peers provides a list of discovered peers in the "full" topic.
// If Discovery hasn't found any peers, it blocks until at least one peer is found.
func (d *Discovery) Peers(context.Context) ([]peer.ID, error)