text: Title: Is it possible to implement custom email validation in AccountAdapter instead of overriding RegisterSerializer.validate_email?
  Body: Hi,
  I'm writing a multitenant app and wanted to use AllAuth.
  However, it [does not have an option to replace `EmailAddress`](https://github.com/pennersr/django-allauth/issues/2450)
  
  I also found this issue pennersr/django-allauth/issues/976 that allows implementing custom logic to validate email uniqueness in `AccountAdapter`, merged in pennersr/django-allauth/pull/1407.
  
  The change in PR updates `BaseSignupForm.clean_email` method. If I understand correctly `dj-rest-auth` `RegisterSerializer` is modelled after `BaseSignupForm`. 
  
  Would it be possible to do the same in `RegisterSerializer` so I do not have to provide my own and can keep the changes only in AccountAdapter?
  