[auth] Allow to specify deeplinks as emailRedirectTo & inside supabase html template builder

[xzilja]

For native applications that want to use supabases magic links for authentication, it would be great if we could provide deeplinks like myapp:// for emailRedirectTo and use them in supabase's email template builder. This would allow us to verify otp within the app and provide a more seamless email to app linking experience. Perhaps some sort of https link can also be specified as fallback in case users open such emails on desktop.

As of now this is the current behaviour:

1. Specifying emailRedirectTo option as myapp:// results in error stating that link needs to be https://
2. Updating email template in supabase to include link like <a href="myapp://{{ .Token}}">Verify now!</a> results in <a>Verify now!</a> when saved (I assume supabase dashbord sees deeplink as invalid and removes it)

If we could at least do number 2, it already solves the issue, but perhaps allowing this on client sdk would lead to a more seamless experience.