fix: autorelay: treat static relays as just another peer source

[MarcoPolo]

fixes #1782 by making static relays a specific type of peer source. Then re-using all the peersource logic. @marten-seemann was there a reason you didn't do this originally?

[marten-seemann]

We probably could be a lot smarter about static relays (for example, doing exponential backoff for the ones we can't connect to), but we can leave that for a future PR.

This logic looks fine, and I'm not sure why I didn't implement it like that back then.

[marten-seemann]

The documentation for WithPeerSource says:

Implementations must send at most numPeers, and close the channel when they don't intend to provide any more peers.

[marten-seemann]

We can use WithMaxCandidaters to make sure that the relayFinder is asking us for enough peers. We should probably also add some documentation that there's little point in providing a huge number of static relays.

[MarcoPolo]

This check is to make sure we send at most numPeers. If we don’t have numPeers we’ll only send what we have. Essentially the min(len(static),numPeers)).

I debated using with WithMaxCandidaters, but your right I’ll add it.

[marten-seemann]

For some reason I thought the if was the other way round (> instead of <). Sorry for that!