Feature request: allow to return value from C++ proxying API

It would be nice to be able to return custom value directly from `queue.proxySync(...)` / `proxySyncWithCtx` / `proxyCallback`'s callback high-level APIs.

Most proxying operations require returning a value, but right now the closest workaround is to create a value with default constructor and override it from the callback, which is a bit cumbersome and also not ideal when default constructor is potentially expensive. Another alternative is to use `std::optional` + `emplace`, which is less expensive, but even more cumbersome.

I'm not sure how that would play with `bool` - I guess `bool` should stay as the return type for `proxySync` and then `std::optional` would be the return type for `proxySync`? Personally I almost always need to hard assert on proxying being successful, so I'd also be happy with something like `T proxySyncChecked()` which would just throw exception / abort on proxying failure and return value directly otherwise.