Support implicit Message Pickup mode as a mediator

[genaris]

Right now, Message Pickup module is only considering V1 and V2 sessions, but there are plenty of Credo-based apps out there (e.g. BC Wallet) that are using Implicit and it would be nice to add native support for them until they are upgraded to use the "clean" V2 Pickup in Live mode.

The so-called implicit mode works like a Live Mode, but it is started without sending any other message than a Trust Ping.

[jamshale]

I'll be working on this. I can't assign myself.

[jamshale]

I opened a PR for this. #2385.