discv5: consider that clients may want to advertise servers

Until there is a reward system for nodes like LES servers, it is not really in the interests of servers to participate in 'proof of time' or 'proof of work' to place ads. Someone running a node may be deterred from enabling these discovery or LES server features.

However, a business developing a consumer application that needs to find servers, for example a mobile phone app running a light client, there is strong incentive for those clients to support each other in whatever ways possible to find the servers.

The onus is essentially on clients to advertise discovered servers (again, as long as there is no reward system in place).

One way of doing this could be for the clients to form their own overlay network and share information privately. Other ways could involve a common, shared scheme. One such common scheme could be the Topic Discovery subsystem: We can consider modifying the protocol so that clients can register servers (or in general, allow ads to be placed about someone else.)