feat: implement first-pass at networking plugin

Implements help, `rebuild`, and `rebuildall` command.

Refs #2403

plugins/network/subcommands/default

@@ -0,0 +1,5 @@
+#!/usr/bin/env bash
+set -eo pipefail; [[ $DOKKU_TRACE ]] && set -x
+source "$PLUGIN_AVAILABLE_PATH/network/internal-functions"
+
+network_help_cmd "network:help"