🔒✉️ feat: allow only certain domain

[berry-13]

Summary

Added a feature to restrict registration to specific domains. This enhancement enhances security by preventing unauthorized users from registering

Change Type

• [x] New feature (non-breaking change which adds functionality)

Testing

https://github.com/danny-avila/LibreChat/assets/81851188/fc14cc5e-ff43-4a07-a482-cb4940cb4583

(in this video the allowed domains were: librechat.ai and gmail.com)

Checklist

• [x] My code adheres to this project's style guidelines
• [x] I have performed a self-review of my own code
• [x] I have commented in any complex areas of my code
• [x] I have made pertinent documentation changes
• [x] My changes do not introduce new warnings
• [x] I have written tests demonstrating that my changes are effective or that my feature works
• [x] Local unit tests pass with my changes
• [x] Any changes dependent on mine have been merged and published in downstream modules.