Implement TrustedGetter::meetup_registry(AccountId, CurrencyIdentifier)

[clangenb]

This getter is missing, but it is needed in the meetup.

[brenzi]

I would prefer if a participant doesn't know the other pseudonyms in advance because that can benefit collusion attacks. But as a temporary simplification we can implement this getter for sure.
We would first need to change the app's meetup flow such that a participant doesn't need to know the registry in advance

[brenzi]

Actually, hiding the registry doesn't prevent collusion attacks anyway, because attackers can correlate meetup locations (which have to be known to the participants for the protocol to work). The only additional exploitable information leaking is how many participants are assigned to the meetup.