Consider: Allow "trust type and subtypes" for serialization

It sounds a bit like the one thing that can be used to exploit things, so i'm worried about allowing this.

We are also wanting to allow "trust the entire module": https://github.com/apple/swift-distributed-actors/issues/547 so that would perhaps be nicer/safer?

Discussed in: https://github.com/apple/swift-distributed-actors/pull/548#discussion_r405661187