feat: add support for routing an app to a specified host:port

This is useful when there is a service not managed by Dokku but should be exposed via the Dokku routing layer. As an example, some binaries (consul, nomad, vault) expose web uis, and are traditionally run on the host directly vs in a container.

Closes #4665

docs/development/plugin-triggers.md

@@ -940,6 +940,21 @@ set -eo pipefail; [[ $DOKKU_TRACE ]] && set -x
 # TODO
 ```
 
+### `network-get-static-listeners`
+
+- Description: Return the network value for an app's property
+- Invoked by: `internally triggered by proxy-build-config`
+- Arguments: `$APP $PROCESS_TYPE`
+- Example:
+
+```shell
+#!/usr/bin/env bash
+
+set -eo pipefail; [[ $DOKKU_TRACE ]] && set -x
+
+# TODO
+```
+
 ### `network-write-ipaddr`
 
 - Description: Write the ipaddr for a given app index