Add ability to disable user registration through environment variable

[askadityapandey]

This PR adds the ability to disable user registration through an environment variable DISABLE_USER_REGISTRATION. If this variable is set to true, the registration endpoint will return a 403 Forbidden status, preventing new users from signing up. This feature can be useful for instances where user registration needs to be temporarily or permanently disabled.

Changes:

1. API Controller (auth.controller.ts):

   • Added a check for DISABLE_USER_REGISTRATION in the register method to prevent user registration if the variable is set to true.

2. Environment Validator (env-validator.ts):

   • Added a validator for the DISABLE_USER_REGISTRATION environment variable to ensure it is correctly handled.

3. Docker Compose (docker-compose.yml):

   • Added DISABLE_USER_REGISTRATION to the environment section to allow configuration of the variable in self-hosted deployments.

Also The project is cool, will look forward to contribute more <3 Fixes #724