chore: Move remaining auth methods to plugins (#4900)

* Move Google, Email, and Azure to plugins

* Move OIDC provider, remove old loading code

* Move AuthLogo to plugin

* AuthLogo -> PluginIcon

* Lazy load plugin settings

plugins/oidc/plugin.json

@@ -0,0 +1,12 @@
+{
+  "name": "OIDC",
+  "description": "Adds an OpenID compatible authentication provider.",
+  "requiredEnvVars": [
+    "OIDC_CLIENT_ID",
+    "OIDC_CLIENT_SECRET",
+    "OIDC_AUTH_URI",
+    "OIDC_TOKEN_URI",
+    "OIDC_USERINFO_URI",
+    "OIDC_DISPLAY_NAME"
+  ]
+}