feat(signup): allow to block signup (#3209)

* feat(signup): allow to block signup

* feat(signup): update environment variable documentation

* test: update auth service tests

* feat(signup): prevent user from reaching out the sign up page

* Fix lint

* Fixes

---------

Co-authored-by: Charles Bochet <charles@twenty.com>

packages/twenty-server/src/integrations/environment/environment.validation.ts

@@ -170,6 +170,11 @@ export class EnvironmentVariables {
   )
   @IsString()
   SENTRY_DSN?: string;
+
+  @CastToBoolean()
+  @IsOptional()
+  @IsBoolean()
+  IS_SIGN_UP_DISABLED?: boolean;
 }
 
 export const validate = (config: Record<string, unknown>) => {