feat: introduce `outgoing-request.into-incoming-request`

[rvolosatovs]

Closes #89

I wish there was a nicer way to handle this, but I am not sure what it could look like at the moment.

This does not support body streaming in a single-threaded context, but at least it allows for invoking incoming-handler.handle

[pchickey]

This issue is complicated and I want to spend time figuring out what we can do to solve it, but I don't believe that we should make this addition to the spec before releasing Preview 2 (0.2.0). Since this is additive, it is potentially a candidate for including in 0.2.1, but right now I want to hold off - sound ok?

[rvolosatovs]

we won't need this in 0.3, so closing for now