feat: download to writer for medias.

[arshamalh]

In case of (b *Bot) Download(file *File, localFilename string) error function, users may need different actions than just saving the file automatically on disk, e.g.

• storing the file on minio, s3, etc.
• piping the file to a request to another service
• piping the file to an AI model
• making the file on the disk however the user wants.

Having another DownloadToWriter method that accepts any kind of io.Writer will help users skip an extra step to save on disk and read from the disk.