Allow user to map custom domain to callback url on Sign with OAuth Provider

[kangmingtay]

Feature request

Partially addresses supabase/supabase#1895 where the user isn't redirected to the redirectTo value specified by the developer after signing in with the desired OAuth provider.

Is your feature request related to a problem? Please describe.

This problem can be broken down into 2 parts:

1. The OAuth provider only recognises that it needs to redirect the user to https://<PROJECT_REF>.supabase.co/auth/v1/callback and shows "Continue to supabase.co" (for google's case)

• Possible solution: The developer will have to map the callback url to a url that they control.

2. Gotrue isn't aware of any urls associated with https://<PROJECT_REF>.supabase.co/auth/v1/callback.

• Possible solution: Implement logic in gotrue to associate custom urls to the callback url.

[kangmingtay]

Duplicate of #142