feat: Add Vietnamese localization

[chuyentt]

Description

This PR adds Vietnamese localization to the firebase_ui_localizations package. It includes translations for various labels, error messages, and button texts to support Vietnamese language users. This enhancement aims to improve the user experience for Vietnamese-speaking users, making the Firebase UI components more accessible and user-friendly in the Vietnamese context.

Related Issues

This PR addresses the need for additional language support in the FirebaseUI-Flutter package, specifically adding support for the Vietnamese language. It does not directly resolve any open issues but contributes to the broader goal of internationalizing the FirebaseUI-Flutter package.

Checklist

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• [x] I read the Contributor Guide and followed the process outlined there for submitting PRs.
• [ ] My PR includes unit or integration tests for all changed/updated/fixed behaviors (See Contributor Guide).
• [ ] All existing and new tests are passing.
• [ ] I updated/added relevant documentation (doc comments with ///).
• [ ] The analyzer (melos run analyze) does not report any problems on my PR.
• [ ] All unit tests pass (melos run test:unit:all doesn't fail).
• [x] I read and followed the Flutter Style Guide.
• [x] I signed the CLA.
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Breaking Change

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• [ ] Yes, this is a breaking change.
• [x] No, this is not a breaking change.