Implement p2079 on top of our senders receivers

[gonidelis]

As we are in the midst of implementing p2300 and new means of asynchronous execution (senders/receivers) with #5045, p2079 adds on top of it by presenting facilities that default the access to system thread pool and ease the usage of custom thread pools. The main puprose is for the senders receivers to carry a default execution context and scheduler.

The two facilites to be implemented are:

• system_context
• system_scheduler

and need to bind properly with our existing senders/receivers implementations.

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