Combine event related function under `setEventListener`

[Mqxx]

Currently we have serialSetErrorCallback and serialSetEventCallback. Both take in a callback function that either gets called on an error or on an event (currently only attached/detached).

The API would be more appealing and useful if we would combine and extend those two functions into one function. For example like this:

MODULE_API auto serialSetEventCallback(
    void (*callback_fn)(
        cpp_core::SerialEventType event_type,
        cpp_core::SerialEventContext *event_context
    )
) -> int;

The cpp_core::SerialEventType could be an enum like this:

/**
 * Possible event types serialSetEventCallback().
 */
enum class SerialEventType : int
{
    kError = 0, ///< An error happened.
    kDeviceDisconnected = 1, ///< An existing device was disconnected from the host.
    kDeviceConnected = 2, ///< A new device was connected to the host.
    kRead = 3, ///< Something was read from the device.
    kWrite = 4, ///< Something was written to the device.
    // ... can be extended if needed in the future
};

The cpp_core::SerialEventContext could be a union struct that represents all possible structs for every possible cpp_core::SerialEventType.

This whole concept could also eliminate the need for the error callback passed into every function, because we have the cpp_core::SerialEventContext that we could pass any data needed for the specific event fired.

For example the struct for the cpp_core::SerialEventType::kError could look like this:

struct Error
{
    int error_code,
    const char *message,
}

With this concept the API would be easy to extend with new features aka. events and such.