Add support for ResponseHeaderModifier for HTTPRouteRule objects

Problem: Users want to add, set, and remove response headers.

Solution: Use add_header and proxy_hide_header NGINX directives to support ResponseHeaderModifier.

If the action is set, we configure the proxy_hide_header directive to remove the header if exists and add_header directive with the given value in the HTTPRoute spec.
If the action is remove, we configure the proxy_hide_header directive to remove the header.
If the action is add, we simply configure the add_header directive with the given value in the HTTPRoute spec.

Author: kevin85421

site/content/overview/gateway-api-compatibility.md

@@ -157,7 +157,8 @@ See the [static-mode]({{< relref "/reference/cli-help.md#static-mode">}}) comman
       - `requestRedirect`: Supported except for the experimental `path` field. If multiple filters are configured, NGINX Gateway Fabric will choose the first and ignore the rest.
       - `requestHeaderModifier`: Supported. If multiple filters are configured, NGINX Gateway Fabric will choose the first and ignore the rest. Incompatible with `urlRewrite`.
       - `urlRewrite`: Supported. If multiple filters are configured, NGINX Gateway Fabric will choose the first and ignore the rest. Incompatible with `requestHeaderModifier`.
-      - `responseHeaderModifier`, `requestMirror`, `extensionRef`: Not supported.
+      - `responseHeaderModifier`: Supported.
+      - `requestMirror`, `extensionRef`: Not supported.
     - `backendRefs`: Partially supported. Backend ref `filters` are not supported.
 - `status`
   - `parents`