Generalise coroutine task<T> type to support passing through receiver context queries to parent receiver/coroutine

We would ideally like to be able to define coroutine tasks that can pass through context from the caller to child coroutines to allow passing things like stop_token, current scheduler, allocator transparently through algorithms implemented as coroutines in the same way that we do for most senders.

Since coroutines are inherently type-erased, this will mean that we need to add the ability to parameterise the `task` type on the set of query CPOs that should be passed through from the parent context.

```c++
template
class task { ... };

template
using cancellable_task = task(get_stop_token)>>;
```

This will probably require hooking up the adaption of senders to the `SenderAwaitable` type to turn them into awaitables in the `await_transform()` rather than relying on `operator co_await()` so that we have the type-information from the promise early enough when defining the coroutine-receiver type to pass to the sender's connect() implementation.

Note that for some CPOs we may need to implement some special logic to adapt/type-erase across the boundary. eg. we might want to adapt from whatever stop-token type the caller has to the target stop-token type by attaching a `stop_callback` to the caller's stop-token. Some investigation will be required to determine what the appropriate strategy should be here.