feat: add coroutine support via ENABLE_CORO cmake flag

[braindigitalis]

This PR adds support for D++ asynchronous coroutines.

For this to work you must:

• Use a C++20 supporting compiler (g++11 onwards etc)
• build and install D++ with the DPP_CORO cmake flag
• build cpp-sdk with the ENABLE_CORO flag

Building with this flag will reveal new functions in the SDK client. Each of the functions such as get_user will have a coroutine equivalent starting with co_ (this is the naming convention of D++, copied from C++ standards). The coroutine equivalent will have no callback parameter and can be awaited.

So, the basic example would become instead:

#include <topgg/topgg.h>
#include <dpp/dpp.h>
#include <iostream>

int main() {
  dpp::cluster bot{"your bot token"};
  topgg::client topgg_client{&bot, "your top.gg token"};

  const auto result = co_await topgg_client.co_get_bot(264811613708746752);
  try {
    const auto topgg_bot = result.get();
    std::cout << topgg_bot.username << std::endl;
  } catch (const std::exception& ext) {
    std::cout << "error: " << ext.what() << std::endl;
  }

  return 0;
}

This is nicer, as it avoids callback hell, and program flow reads in a nicer way. This is of course completely optional and if you don't want to use coroutines, or if you can't, the existing API calls will work exactly as before.

[null8626]

Thank you sooo much for this! I am not familiar with C++ coroutines hahahaha so this pull request is a huge blessing! ❤️