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

frontend/js/app/settings/list/item.ejs

@@ -9,6 +9,14 @@
         <% if (id === 'default-site') { %>
             <%- i18n('settings', 'default-site-' + value) %>
         <% } %>
+        <% if (id === 'oidc-config' && meta && meta.name && meta.clientID && meta.clientSecret && meta.issuerURL && meta.redirectURL) { %>
+            <%- meta.name %>
+            <% if (!meta.enabled) { %>
+               (Disabled)
+            <% } %>
+        <% } else if (id === 'oidc-config') { %>
+            Not configured
+        <% } %>
     </div>
 </td>
 <td class="text-right">