Add a bypass scheduler to improve mpi continuations

[biddisco]

I would like this to go through testing ASAP, so I'm pushing it, but it can be can be considered a draft PR for now Relies on #1151 and #1180

Here, we add a stdexec style scheduler that executes a task/function - it behaves like the main pika scheduling loop - by takinf the function, wrapping it in a task, then switching context to it immediately, so that if the task later tries to suspend, then there are no problems and the task goes ontoo the queues as usual.

It is an error to call this on a pika thread and is only supported by certain modes of the mpi continuations