Add custom logger capabilities

[bwrsandman]

Add levels to public API. Default to old stderr logging method. Add function pointer and userdata pointer as static globals.

Implements #179

[bwrsandman]

Example use of custom logger using spdlog in c++

void unshield_log_handler([[maybe_unused]] void* userdata, int level, const char* file, int line, const char* format, va_list args)
{
    spdlog::level::level_enum spdlogLevel;
    switch (level)
    {
    case UNSHIELD_LOG_LEVEL_TRACE:
        spdlogLevel = spdlog::level::trace;
        break;
    case UNSHIELD_LOG_LEVEL_WARNING:
        spdlogLevel = spdlog::level::warn;
        break;
    case UNSHIELD_LOG_LEVEL_ERROR:
        spdlogLevel = spdlog::level::err;
        break;
    }

    // Get the length of the message
    va_list args_copy;
    va_copy(args_copy, args);
    int length = vsnprintf(nullptr, 0, format, args_copy) + 1;
    va_end(args_copy);

    std::vector<char> message(length);
    vsnprintf(message.data(), message.size(), format, args);

    SPDLOG_LOGGER_CALL(spdlog::get("filesystem"), spdlogLevel, "unshield: {}", message.data());
}

[twogood]

@bwrsandman sorry about the merge conflict!

[bwrsandman]

rebased!