Add IP Whitelist Plugin and corresponding tests

- Introduced `IpWhitelistPlugin` (DOMAIN): Restricts access to specific IP addresses or CIDR ranges.
- Added configuration options: `enabled`, `allowed_ips`, and `status_code`.
- Updated `plugins.md` and `plugin-development.md` to document `IpWhitelistPlugin`.
- Created fixtures for `services-with-ip-whitelist`.
- Added extensive test cases for `IpWhitelistPlugin` to validate configuration and HAProxy output generation.
- Ensured seamless integration into the plugin framework alongside other domain plugins.

src/tests/fixtures/services-with-ip-whitelist

@@ -0,0 +1,10 @@
+{
+    "192.168.1.40": {
+        "easyhaproxy.http.host": "secure.example.com",
+        "easyhaproxy.http.port": "80",
+        "easyhaproxy.http.localport": "8080",
+        "easyhaproxy.http.plugins": "ip_whitelist",
+        "easyhaproxy.http.plugin.ip_whitelist.allowed_ips": "192.168.1.0/24,10.0.0.5",
+        "easyhaproxy.http.plugin.ip_whitelist.status_code": "403"
+    }
+}