Add email attribute to the Client class

[adiroiban]

Let's encrypt no longer sends email notification

https://letsencrypt.org/2025/06/26/expiration-notification-service-has-ended.html

but the email is still part of the API.

In #164 I have removed the email part from the client ... but not from the server...so the code in trunk is broken

The email functionality should be added back.

Maybe you want to use txacme with a custom Acme server that supports notifications