Generalize condition_variable wakeup

[xemul]

For the past years we had several attempts from both internal and external contributors to add abort-source as a way to wakeup c.v. from sleep.

The main concern against adding it is to avoid a matrix of every primitive interacting with every other primitive. There is a workaround

auto abort_me = as.subscribe([&] { cv.broadcast(); });
...
co_await cv.when([&] { return as.abort_requested() || other_conditions; });

but it's less than nice and optimal.

Apparently, it's needed badly, but the consensus is yet to be come to.

[xemul]

refs: redpanda-data/redpanda#27845

[avikivity]

Perhaps we should add a customization mechanism.

Let's say the user calls cv.when(a1, a2, a3).

If the expression condition_variable_awaiter(a1) compiles, then we instantiate the result (that's the abort_me thing, let's call it x) and check x() in the conditional.

We could supply awaiter transforms for std::invocable<>s, abort_source:s, various time_point:s (for timeouts) etc.

[rockwotj]

Why does a semaphore take an abort source? Is that a higher level primitive than a conditional variable?

[avikivity]

It is not.