Add a cover image for channels

When creating either a group or an open channel, you can add a cover image by specifying an image URL or file. You can fetch the cover image URL using the cover_url property. You can also update a channel's cover image by calling UpdateChannel().

SBDOpenChannel::CreateChannel(NAME, CHANNEL_URL, COVER_URL, DATA, OPERATOR_USER_IDS, CUSTOM_TYPE, [](SBDOpenChannel* openChannel, SBDError* error) {
    if (error != nullptr) {
        // Handle error.
        return;
    }
});