refactor: deprecate proxy-configure-ports plugin trigger in favor of ports-configure

This also rewrites the logic to be in golang, which will make it easier to refactor portions of it to use the property system going forward.

plugins/common/ports-get-available

@@ -0,0 +1,6 @@
+#!/usr/bin/env bash
+source "$PLUGIN_CORE_AVAILABLE_PATH/common/functions"
+set -eo pipefail
+[[ $DOKKU_TRACE ]] && set -x
+
+get_available_port "$@"