Add support for email confirmation codes

[jperezr21]

Goals

1. Allow users to verify their email by sending them a confirmation code, instead of a Magic Link
2. Avoid this issue:

• Email provider on iOS: session-token not passed from gmail safari web view to safari browser app #4524
• Somehow handle magic links with Gmail's in-app browser (or similar) devise-passwordless/devise-passwordless#61
• https://github.com/orgs/supabase/discussions/15708

Non-Goals

No response

Background

Magic Links don't work on iOS when using the Gmail app. Link are opened in Gmail's in-app browser, which doesn't share sessions with Safari. Users can't even copy the links, because long-pressing them opens a preview, which uses the token and makes it expired.

Proposal

In addition to Magic Links, confirmation codes should be supported as an alternative.