feat: Add a `linkAccountOverride` method in adapter

[Khaaz]

☕️ Reasoning

This allows to better control the authentication flow. Effectively, it exposes a linkAccountOverride method in the adapter.

This method will be triggered through this workflow:

• a user A is created through signing in with account A
• a user B is created through signing in with account B
• connected with account A (and user A), signing in with account B

Actual behaviour:

• sends back an error saying it is not possible to link an account already linked to a different user

Expected behaviour:

• Allow to customise what we want to do in this situation

What this PR offers:

• a method to control what to do in this situation, potentially allowing to link account B to user A

🧢 Checklist

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

🎫 Affected issues

Fixes: https://github.com/nextauthjs/next-auth/issues/9339

📌 Resources

• Security guidelines
• Contributing guidelines
• Code of conduct
• Contributing to Open Source

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[Khaaz]

I'll gladly add tests or docs, please let me know if this feature is acceptable first.

I would also love quick guidance on documentation or tests if possible (I'll try to figure it out anyway after confirmation this PR has a future).

Looking forward to any feedback as I definitely need this feature for my usecase.

Thanks a lot.