[shove70]

This PR: Implement smtp mail attachment support.
Close #19920

send_cfg := smtp.Mail{
		from: '${nicename}<${sender}>'
		to: receivers
		subject: subject
		body_type: smtp.BodyType.html
		body: body
		attachments: [
			smtp.Attachment {
				filename: 'readme.txt'
				bytes: '...Some file content...'.bytes()
			}
		]
	}

	mut client := smtp.new_client(client_cfg)!
	client.send(send_cfg)!
	client.quit()!