feat: Add support for custom username overrides

[mathieu-lemay]

Add a new option to manually define the usernames for selected email addresses. This is mostly useful for OAuth providers that don't return preferred_username.

I'm opening this as a draft since it's a quick and dirty fix. If this is something you're interested in, I will take the time to add some validations (like email / username is not empty) and some tests.

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[steveiliop56]

@mathieu-lemay thank you for the pull request.

I am a bit skeptical about it. Why add another file, another field to manage and another mapping while we already have user attributes? Can't the same outcome be achieved by adding a new PreferredUsername field in the UserAttributes struct? Granted, it will require the user attributes to also mess with OAuth users but I believe it's a better idea because what if we also want a custom name for OAuth users in the future? Another file?

[mathieu-lemay]

@steveiliop56 Thanks for the reply, this is exactly the sort of feedback I wanted! I didn't think of using UserAttributes because I haven't used that feature in my workflow with tinyauth. I'll look into it and get back to you