Allow adapter specific parts

[patriksvensson]

It would be nice if you could register parts that's only used by certain adapters. The parts would then get a ReplyContext<TBroker> instead of the non-generic ReplyContext.

[agc93]

This would be very handy!

It would also mean adapters could potentially expose extra public members on the TBroker for adapter-specific functionality, no?

[patriksvensson]

@agc93 Exactly.