Add protocol support and improve FastCGI plugin handling

- Introduced `easyhaproxy.proto` annotation for backend protocol configuration.
- Automatically set FastCGI protocol (`proto: fcgi`) when using the FastCGI plugin.
- Updated Kubernetes processor to handle `proto` annotations.
- Enhanced EasyMapping to allow plugin overrides for host-level configs.
- Updated documentation to reflect new `proto` behavior and FastCGI improvements.
- Improved multiprocessing configuration with better context handling in `haproxy.py`.

src/plugins/builtin/fastcgi.py

@@ -148,7 +148,7 @@ class FastcgiPlugin(PluginInterface):
 
         return PluginResult(
             haproxy_config=backend_config,  # use-fcgi-app directive for the backend
-            modified_easymapping=None,
+            modified_easymapping={"proto": "fcgi"},
             metadata=metadata,
             global_configs=[fcgi_app_definition]  # For top-level injection
         )