C++20 coroutines for ESPHome core

[scaiper]

Hi,

I've recently made a component utilizing an UART bus to communicate with a device. A device itself is not very fast to respond, a communication session can take up to hundreds of milliseconds. The Contributing Guide recommends making a state machine in this case.

So I've tried to make a state machine using C++ coroutines and I'm quite happy with the result. The coroutines are convenient to use, and are quite transparent.

Inside a coroutine you can co_await another coroutine, they will form a stack as one would expect. Or you can co_await an instance of std::suspend_always to suspend until a next loop iteration.

An example of a coroutine reading from UART.

Coroutine<bool> ZehnderComfoAirComponent::read_array_coro(Context&, uint8_t* data, size_t data_len) {
  while (static_cast<size_t>(this->available()) < data_len) {
    co_await std::suspend_always{};
  }

  co_return this->read_array(data, data_len);
}

There is also a sequential scheduler to queue up invocations so they will run strictly sequentially and not interleaved. This queue in intended to be per component, so that synchronization inside a component is not needed.

As C++20 support has landed recently, I think, this coroutines would be useful addition to the ESPHome core, as this kind of async execution can be often needed. I'd like to hear some opinions.

If there are no objections, I can work on the PR with changes.