Add support for CORS in HAProxy stats dashboard

- Introduced `HAPROXY_STATS_CORS_ORIGIN` environment variable for enabling CORS in HAProxy stats.
- Updated HAProxy configuration template to handle CORS preflight and response headers.
- Added tests for static configuration mode with CORS enabled.
- Updated documentation to include new CORS configuration details and examples.

tests/fixtures/static_cors.yml

@@ -0,0 +1,11 @@
+stats:
+  username: admin
+  password: test123
+  port: 1936
+  cors_origin: http://localhost:3000
+
+customerrors: false
+
+containers:
+  "host1.com.br:80":
+    ip: ["container:5000"]