* feat: allow only certain domain
* Update dotenv.md
* refactor( registrationController) & handle ALLOWED_REGISTRATION_DOMAINS not specified
* cleanup and moved to AuthService for better error handling
* refactor: replace environment variable with librechat config item, add typedef for custom config, update docs for new registration object and allowedDomains values
* ci(AuthService): test for `isDomainAllowed`
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Co-authored-by: Danny Avila <messagedaniel@protonmail.com>
