feat: coroutines "beta test"

[Mishura4]

dpp::task - coroutine object

Hot start (starts on constructor), executes in parallel
Can be cancelled which will throw dpp::task_cancelled_exception after the next co_await
Cannot detach, task will cancel itself when destroyed
Can be co-awaited later to retrieve return value
Requires synchronization at the point of co_await and destruction (lock-free)

dpp::coroutine - coroutine object

Lazy start (starts on co_await)
Has a return value, retrieved on co_await
Cannot detach or run in parallel by design of lazy start
No synchronization, light object

dpp::job - coroutine object

Hot start, runs in parallel and detached always, destroys itself when done
Cannot be co_awaited
No synchronization, no return value, very light object

dpp::async - awaitable API request (previously dpp::awaitable)

Can be constructed from a function
e.g:

• returned by co_*: dpp:async request = cluster->co_message_create(dpp::message{"foobar"});
• from arbitrary function: dpp::async request{cluster, &dpp::cluster::message_create, dpp::message{"foobar"}};

Hot start (starts on constructor), executes in parallel
Lock-free synchronization on co_await

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[Mishura4]

Depends on #775 as one of the examples in the docs gets a SIGSEGV from the destructor of a copied dpp::emoji
✅ Merged

[CLAassistant]

All committers have signed the CLA.

[wizard7377]

Please wrap all C++20 specific features in a #if __cplusplus != 201703L and #endif

[Mishura4]

DPP_CORO covers that.

[Mishura4]

This is finally ready for review. 👍

[Mishura4]

Btw I suggest merging this PR normally, without squash, so each changes gets listed in the changelog one by one