Let users opt-in to mailing list when they enroll an email address

We have a Mailchip email list, which we would like to use to send users updates about the app.

In the UI for the m.enroll.email.request_token stage, we should add a checkbox where the user can opt-in to receive email updates.

Then in the UIA request for that stage, we should include a new boolean value to indicate whether the user wants to be added to the email list.

{
    ...
    "auth": {
        "session": "abcdwxyz",
        "type": "m.enroll.email.request_token",
        "email": "user@domain.tld",
        "subscribe_to_list": true,
    }
}