[Senders/Receivers] `as_awaitable`

[SAtacker]

Expected Behavior

All awaitables are senders Quoting from P2300

as_awaitable is used to transform an object into one that is awaitable within a particular coroutine

This function needs to be implemented for Senders/Receivers

Usage

To make non-awaitable senders as awaitable.

Possible cues to implementation

type sender-awaitable as described in the above P2300 and type checking for Senders using templates to check if they have co_await method/free operator

CC @hkaiser

[SAtacker]

For the case when the sender is not an awaitable we should have a base for awaitable which consists of a coroutine_handle, promise object defined in libs/core/futures/include/hpx/futures/promise.hpp, and hpx::execution::operation state defined in libs/core/execution/include/hpx/execution/algorithms/just.hppif I am not wrong?