[API] subscribers endpoints based on email instead of id

[Seb33300]

Hello,

I think it could be more convenient to use the email to reference a subscribers in the API endpoint.
email is unique so it can be used instead of id.
(This is how API endpoints work with SendGrid, Mailjet and others...)

Because the API is intended to be used between 2 different application, and my other application has no way to know the subscriber id.