Gossip public Torii URLs to help with automatic discovery

[0x009922]

Origin: #5348 (comment)

The issue is that for Explorer to be useful (specifically, to display telemetry page), it needs to gather info not only from a single peer in the network, but from all of them (or at least many of them). Currently the only way to do so is to give Explorer all peers Torii URLs explicitly. This doesn't scale and doesn't handle dynamic peers.

There is network.public_address, but it is used for P2P and has nothing to do with the public URL on which the peer's Torii is available. And it isn't just socket address (ip/host + port). In real world, it absolutely can have a path component as well (e.g. http://fujiwara.sora.org/v5/).

My proposal is to:

• add torii.public_url config parameter (optional)
• gossip this info among peers, in a form of just FYI
• change GET /peers, return peer ids + their public URLs.