Add a version for response promises that accept an expected

[Neverlord]

The idea is to support something like self->mail(foo).request(bar, 1s).then([](expected<my_type> res) { ... }). If foo is statically typed, we also could infer the type to allow self->mail(foo).request(bar, 1s).then([](auto res) { ... }).

I do have a POC for this, but I'm not happy with it. There's a lot of guessing and meta programming indirections involved that make it rather hard to see what's going on, since the implementation has to fall back to the "regular" code if the handler does not take an expected. A way around this could be to have a separate name, e.g., then_ex, to reduce the complexity of the implementation. Won't be as nice, but at least the user would explicitly opt-in to the alternative implementation.