refactor: split out proxy port configuration into it's own trigger

This will allow it to be called by plugins other than the nginx-vhosts one.

docs/development/plugin-triggers.md

@@ -1842,6 +1842,21 @@ set -eo pipefail; [[ $DOKKU_TRACE ]] && set -x
 # TODO
 ```
 
+### `proxy-configure-ports`
+
+- Description: Configures the proxy port mapping
+- Invoked by: `internally triggered by proxy plugins`
+- Arguments: `$APP`
+- Example:
+
+```shell
+#!/usr/bin/env bash
+
+set -eo pipefail; [[ $DOKKU_TRACE ]] && set -x
+
+# TODO
+```
+
 ### `proxy-disable`
 
 - Description: Disables the configured proxy implementation for an app