Add "share_UNL" to config, exposing the currently used validators in /crawl

[MarkusTeufelberger]

While it would be a rather informal way to share your UNL (and could of course be faked etc.), it still would give at least some hints about validator topology which would be needed for decentralizing.

Additionally, a signature with your validator's key might be added, to give some more concrete proof that if the node fakes this statement, it is done knowingly and deliberately.

[nbougalis]

Ping @garay-daniel.

[MarkusTeufelberger]

UNLs can be exposed via /crawl now (and by default are...). They don't carry signatures though, so they are fairly easy to plausibly deny.

Since the current implementation fulfills the ticket's title, I'll close this.

@garay-daniel, feel free to comment or reopen.