Provide a dedicated way of constructing a runtime that owns exactly one worker thread

[jplatte]

Is your feature request related to a problem? Please describe.

I was looking to switch from a multithreaded runtime to a single-threaded one, and naively I assumed I could work with what's provided with just the rt feature (no rt-multi-thread). After lots of docs reading and a discussion on Discord I now know that this is not correct.

Describe the solution you'd like

I would like there to be another constructor for runtime::Builder, for a runtime that owns exactly one worker thread. I don't know much about the runtime internals so don't know how this could interact with the existing runtime Cargo features, but even if it would only be available under rt-multi-thread, it could still help people understand that they should not use the current_thread runtime if they really want the runtime to conceptually own one worker thread.

[Darksonn]

This was previously discussed in #2720.