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/google/plugin.json

@@ -0,0 +1,5 @@
+{
+  "name": "Google",
+  "description": "Adds a Google authentication provider.",
+  "requiredEnvVars": ["GOOGLE_CLIENT_ID", "GOOGLE_CLIENT_SECRET"]
+}