Allow Requests to be Mirrored to different backends

Add a feature which allows traffic to be mirrored to
additional backends. This is useful for testing how
requests will behave on different "test" backends.

See https://nginx.org/en/docs/http/ngx_http_mirror_module.html

rootfs/etc/nginx/template/nginx.tmpl

@@ -999,6 +999,11 @@ stream {
             {{ opentracingPropagateContext $location }};
             {{ end }}
 
+            {{ if $location.Mirror.URI }}
+            mirror {{ $location.Mirror.URI }};
+            mirror_request_body {{ $location.Mirror.RequestBody }};
+            {{ end }}
+
             rewrite_by_lua_block {
                 lua_ingress.rewrite({{ locationConfigForLua $location $server $all }})
                 balancer.rewrite()
@@ -1091,7 +1096,6 @@ stream {
             }
             {{ end }}
 
-
             {{ if not $location.Logs.Access }}
             access_log off;
             {{ end }}