FEAT: Add Open ID Connect authentication method

* add `oidc-config` setting allowing an admin user to configure parameters
* modify login page to show another button when oidc is configured
* add dependency `openid-client` `v5.4.0`
* add backend route to process "OAuth2 Authorization Code" flow
  initialisation
* add backend route to process callback of above flow
* sign in the authenticated user with internal jwt token if internal
  user with email matching the one retrieved from oauth claims exists

Note: Only Open ID Connect Discovery is supported which most modern
Identity Providers offer.

Tested with Authentik 2023.2.2 and Keycloak 18.0.2

backend/routes/api/tokens.js

@@ -28,6 +28,8 @@ router
 			scope:  (typeof req.query.scope !== 'undefined' ? req.query.scope : null)
 		})
 			.then((data) => {
+				// clear this temporary cookie following a successful oidc authentication
+				res.clearCookie('npm_oidc');
 				res.status(200)
 					.send(data);
 			})