feat: read username if email is not available for oauth provider

Also add recipe for OAuth with GitHub setup

recipes/oauth/nginx.conf

@@ -0,0 +1,18 @@
+server {
+    listen 80;
+
+    location /repo {
+        rewrite ^/repo/(.*) /$1 break;
+        autoindex on;
+        root /srv/ahriman/repository;
+    }
+
+    location / {
+        proxy_set_header Host $host;
+        proxy_set_header X-Real-IP $remote_addr;
+        proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
+        proxy_set_header X-Forwarder-Proto $scheme;
+
+        proxy_pass http://backend:8080;
+    }
+}