Allow further customization of generateVerificationToken in email provider

[THE-SIMPLE-MARK]

Goals

1. A way to use your own encryption settings, with your chosen modules, etc.
2. A way to do other tasks related to the verification token creation in one operation
3. Overall further customizability of the email authentication flow.

Non-Goals

The goals should be self explamantory. I think this part is unnecessary.

Background

So, my story: I was setting up the magic email link provider for my website, when I realized that I should probably add a backup solution for when the link doesn't work for whatever reason. This is good and all, but with the prisma provider, I can't generate this code when the verification token is created (without modifying the db provider; which I consider to be bad practise).

So, I have to use a sleep function in the sendVerificationRequest function to wait a 100 milliseconds and then modify the freshly created verification token with the shorter, human readable code. This is code is then sent in the same email as the magic link, but as plain text.

I don't see any other better alternatives.

Proposal

Expand the generateVerificationToken function so everything returned from it is sent directly to the sendVerificationRequest function. This would require the removal of most of the hardcoded logic from the sendToken function.