refactor: move external procfile manipulation into triggers

This also cleans up the internal APIs a bit.

docs/development/plugin-triggers.md

@@ -1447,6 +1447,51 @@ APP="$1";
 curl "https://dokku.me/starting/${APP}" || true
 ```
 
+### `procfile-extract`
+
+- Description: Extracts a Procfile from a given image to a path
+- Invoked by: `internally`
+- Arguments: `$APP $IMAGE $FORCE_EXTRACT $PROCFILE_PATH`
+- Example:
+
+```shell
+#!/usr/bin/env bash
+
+set -eo pipefail; [[ $DOKKU_TRACE ]] && set -x
+
+# TODO
+```
+
+### `procfile-get-command`
+
+- Description: Fetches the command for a specific process type
+- Invoked by: `internally`
+- Arguments: `$APP $PROCESS_TYPE $PORT $PROCFILE_PATH`
+- Example:
+
+```shell
+#!/usr/bin/env bash
+
+set -eo pipefail; [[ $DOKKU_TRACE ]] && set -x
+
+# TODO
+```
+
+### `procfile-remove`
+
+- Description: Removes the extracted Procfile
+- Invoked by: `internally`
+- Arguments: `$APP $PROCFILE_PATH`
+- Example:
+
+```shell
+#!/usr/bin/env bash
+
+set -eo pipefail; [[ $DOKKU_TRACE ]] && set -x
+
+# TODO
+```
+
 ### `proxy-build-config`
 
 - Description: Builds the proxy implementation configuration for a given app