break: remove co_attach & enforce event handler call semantics (return void/dpp::job, take const event)

[Mishura4]

Context (accessible to reviewers)
For non-reviewers, the tl;dr is std::function does very little to enforce call semantics such as a return type.
This PR actually enforces the call semantics using type traits. It also removes co_attach and now unifies usage for coro / non coro event handlers (you attach a coro event handler using attach or operator()).

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[braindigitalis]

don't the coro docs need to be updated to reflect this change? please dismiss review if this will be a separate pr

[Mishura4]

TRUE i forgor, nice catch. Will update tomorrow

[Mishura4]

Updated the docs (diff here)

[Mishura4]

This will likely conflict with #808 as well, if it's merged first I'll resolve them on my end