Add coroutine support functions to message_create_t (co_send, co_reply)

[Feinq]

The message_create_t type was missing direct coroutine function variations. So I have implemented functions and overloads for both co_send() and co_reply() functions. This PR improves code consistency and usability.

(Similar coroutine functions have already been implemented for interaction_create_t before, but not for message_create_t)

Code change checklist

• I have ensured that all methods and functions are fully documented using doxygen style comments.
• My code follows the coding style guide.
• I tested that my change works before raising the PR.
• I have ensured that I did not break any existing API calls.
• I have not built my pull request using AI, a static analysis tool or similar without any human oversight.

Single-File Test Bot

#include <dpp/dpp.h>
#include <dpp/coro.h>
#include <coroutine>
#include <iostream>

const std::string BOT_TOKEN = "your bot token here";

int main() {
    dpp::cluster bot(BOT_TOKEN, dpp::i_default_intents | dpp::i_message_content);

    bot.on_log(dpp::utility::cout_logger());

    /*
    This example demonstrates the use of co_reply and co_send functions on the dpp::message_create_t event.
    You can test them by sending !test1, !test2, !test3, and !test4 in a channel the bot is in.
    The first 3 tests each perform a co_reply with different overloads, the 4th test performs a co_send with all overloads.
    */

    bot.on_message_create([](const dpp::message_create_t& event) -> dpp::task<void> {

        std::cout << "Received message: " << event.msg.content << std::endl;

        if (event.msg.content == "!test1") {
            dpp::confirmation_callback_t result = co_await event.co_reply("This is a test reply.", true);
            if (!result.is_error()) {
                std::cout << "1st co_reply with string succeeded!" << std::endl;
            } else {
                std::cout << "1st co_reply with string failed: " << result.get_error().message << std::endl;
            }
        }

        if (event.msg.content == "!test2") {
            dpp::message msg("This is a test reply.");
            dpp::confirmation_callback_t result = co_await event.co_reply(msg, false);
            if (!result.is_error()) {
                std::cout << "2nd co_reply with message obj succeeded!" << std::endl;
            } else {
                std::cout << "2nd co_reply with message obj failed: " << result.get_error().message << std::endl;
            }
        }

        if (event.msg.content == "!test3") {
            dpp::message msg("This is a test reply.");
            dpp::confirmation_callback_t result = co_await event.co_reply(std::move(msg), true);
            if (!result.is_error()) {
                std::cout << "3rd co_thinking with moved message obj succeeded!" << std::endl;
            } else {
                std::cout << "3rd co_thinking with moved message obj failed: " << result.get_error().message << std::endl;
            }
        }

        // Test all of the co_send overloads
        if (event.msg.content == "!test4") {
            dpp::confirmation_callback_t result1 = co_await event.co_send("This is a test send.");
            if (!result1.is_error()) {
                std::cout << "1st co_send with string succeeded!" << std::endl;
            } else {
                std::cout << "1st co_send with string failed: " << result1.get_error().message << std::endl;
            }

            dpp::message msg("This is a test send.");
            dpp::confirmation_callback_t result2 = co_await event.co_send(msg);
            if (!result2.is_error()) {
                std::cout << "2nd co_send with message obj succeeded!" << std::endl;
            } else {
                std::cout << "2nd co_send with message obj failed: " << result2.get_error().message << std::endl;
            }

            dpp::message msg2("This is a test send.");
            dpp::confirmation_callback_t result3 = co_await event.co_send(std::move(msg2));
            if (!result3.is_error()) {
                std::cout << "3rd co_send with moved message obj succeeded!" << std::endl;
            } else {
                std::cout << "3rd co_send with moved message obj failed: " << result3.get_error().message << std::endl;
            }
        }

    });

    bot.start(dpp::st_wait);
}```

[CLAassistant]

All committers have signed the CLA.

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[braindigitalis]

lgtm