[Discuss] Support stored scripts in Fleet packages

`package-spec` currently only supports inline scripts. Larger, more sophisticated scripts tend to be used as stored scripts instead of inline. [Stored scripts](https://www.elastic.co/guide/en/elasticsearch/reference/master/modules-scripting-using.html#script-stored-scripts) are also known to reduce compilation time. This issue is to discuss if loading stored scripts should be supported in package-spec.

cc: @ruflin