Revisit: Wallet messaging

[jrriehl]

trafficstars

Revisit wallet message handlers, in particular, whether we want to allow multiple handlers for messages, overwrite them, or discriminate based on the messages in some way.

[Archento]

The wallet messaging client is part of the context and should be more closely aligned with the current implementation in Agentverse. It makes sense to relocate the client or at least rethink how we handle wallet messaging in uAgents.