feat: add `isNewUser` arg on the signIn callback

[kahveciderin]

This PR implements the isNewUser field on the signIn callback, to allow for the rejection of a potential sign up.

☕️ Reasoning

Sometimes we might want to reject the signIn event if the user doesn't already have an account.

🧢 Checklist

• [x] Documentation
• [ ] Tests
• [x] Ready to be merged

🎫 Affected issues

Fixes #8272

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[kahveciderin]

As far as I understand, the "Credentials" provider is intended to support use cases where one has an existing system one needs to authenticate users against, so a user cannot be a new user when using the Credentials provider, thus I've now added isNewUser: false to the signIn callback when signing in with the credentials provider to reflect that.

[kahveciderin]

I have exported the "input validation" lines to an external helper function, because the same thing with the callbackHandler was being validated in the new checkIfUserIsNew function. Also, I simplified the checkIfUserIsNew function's logic by first checking the email no matter what provider is used, and it checks if the account exists only after establishing that the email doesn't exist and the user is using oauth.

[kahveciderin]

I think I forgot to stage my last changes, will be pushing them now.

[kahveciderin]

This PR should now be ready to merge, would be happy to get some reviews.

[april83c]

What's going on with this? Will it be merged?

[bilalmohib]

Please merge this work

[ehenoma]

push

[siddharthd0]

merge when

[garshythoel]

Another bump here! Thanks

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[stale[bot]]

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