Chore: Group auth docker blocks (#54274)

* Move auth docker images to parent folder

* Add README file for auth parent folder

* Add grafana-authnz-team as code owner

* Update `devenv` docs

devenv/docker/blocks/auth/prometheus_basic_auth_proxy/nginx.conf

@@ -0,0 +1,35 @@
+events { }
+
+http {
+  server {
+    error_log stderr info;
+
+    listen 10090;
+
+    location / {
+
+      # Removes any Access-Control-Allow-Origin from Prometheus itself. When accessing from browser, having * or
+      # multiple values is not allowed in some cases
+      proxy_hide_header Access-Control-Allow-Origin;
+
+      # Allow the origin access. This is kinda wildcard but for browser it seems more strict and is needed for
+      # withCredentials requests.
+      add_header Access-Control-Allow-Origin $http_origin;
+
+      # When using withCredentials requests this must be true.
+      add_header Access-Control-Allow-Credentials true;
+
+      # Ask for basic auth except for pre flight OPTIONS request.
+      limit_except OPTIONS {
+        ################################################################
+        # The htpasswd file contains user:
+        # prom: test
+        ################################################################
+        auth_basic "prom";
+        auth_basic_user_file /etc/nginx/htpasswd;
+      }
+
+      proxy_pass http://prometheus:9090/;
+    }
+  }
+}