Add ability to logout from OIDC (#6539)

* Add OIDC_LOGOUT_URI functionality

* Add logout redirect to work on logout route

* Fix lint

* Fix lint

* Fix lint

* Fix lint

* Return null if logout endpoint used

* Update import

server/env.ts

@@ -499,6 +499,16 @@ export class Environment {
     process.env.OIDC_USERINFO_URI
   );
 
+  /**
+   * The OIDC logout endpoint.
+   */
+  @IsOptional()
+  @IsUrl({
+    require_tld: false,
+    allow_underscores: true,
+  })
+  public OIDC_LOGOUT_URI = this.toOptionalString(process.env.OIDC_LOGOUT_URI);
+
   /**
    * The OIDC profile field to use as the username. The default value is
    * "preferred_username".