Support invocation result subscription
Make it possible for the outgoing_call to subscribe for the invocation result. From the module developer's perspective, it should be easy to create such invocation and either save it somewhere for future cancellation or even dispose of it. Then, the kernel should call back into this module to notify it that the invocation is completed (either successfully or with an exception). From the module developer's perspective, there should be a way to understand which exact invocation was completed and receive a return value or an exception (like when calling outgoing_call::finish or outgoing_call::wait), like with the std::future. Make sure that no additional module-to-kernel calls are used for subscription, existing invocation initializing calls should be extended instead.
Also, read https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2022/p2300r5.html#design-pipeable when doing so to make sure the API is easy to use.