Allow to disable SMTP completely

Real production envs will ensure to user their own mailing system. The supabase mails are good to get started but they aren't as flexible in terms of layout etc which is okay.

However handling ones own mails also means: I don't want anyone to skip those and manipulate supabase to use the supabase mailing system. I can do that by adding an SMTP that simply does nothing.

Or even easier: Allow me to DISABLE SMTP completely such that no mails will get sent at all.