refactor: move app.json script execution into it's own plugin

Rather than have this live in 00_dokku-standard, move it out so that it may start to support various other features around the app.json manifest.

plugins/app-json/pre-deploy

@@ -0,0 +1,18 @@
+#!/usr/bin/env bash
+set -eo pipefail
+[[ $DOKKU_TRACE ]] && set -x
+source "$PLUGIN_CORE_AVAILABLE_PATH/common/functions"
+source "$PLUGIN_CORE_AVAILABLE_PATH/app-json/internal-functions"
+
+app_json_pre_deploy() {
+  declare desc="app-json scripts execution"
+  local trigger="app_json_pre_deploy"
+  local APP="$1"
+  local IMAGE_TAG="$2"
+  local PHASE_SCRIPT_KEY="predeploy"
+
+  dokku_log_info1 "Attempting to run scripts.dokku.$PHASE_SCRIPT_KEY from app.json (if defined)"
+  execute_script "$APP" "$IMAGE_TAG" "$PHASE_SCRIPT_KEY"
+}
+
+app_json_pre_deploy "$@"