Support i18n (at least for custom email)

[fromi]

Summary of proposed feature
I try to use sendVerificationRequest in the email provider configuration to send a custom email. I want to email to depend on the user language, however there is no way to pass some parameters, from exemple from the initial POST request, down to sendVerificationRequest function.

Purpose of proposed feature
Send an email with a magic link in the language of my user

[fromi]

Right now I have a super ugly workaround: I use the redirect callback to parse the redirection URL, extract the language from that URL and save it in a global variable to reuse it in the sendVerificationRequest function 😱
Is there a better way to do it? I expect of course collisions between my users once in production...

[fromi]

I created a pull request that I think should allow for more customization: #831