Create checklist for adding new assets to packages

Today packages support a basic list of assets for the Elastic Stack. Moving forward more assets like transform (https://github.com/elastic/package-spec/issues/23) need to be supported. Any addition of an assets needs changes in multiple places and similar questions need to be answered. This issue is to discuss and create a checklist for adding new assets.

Here a first attempt on what needs to be checked / discussed for each asset:

* Package spec
* What is the format of the assets in the package
* What is the path the assets is stored under
* Does it support any special configuration
* Naming of the files and supported formats (for example json vs yaml)
* Elasticsearch Asset
* How is the asset created?
* Does the asset to be created require any previous assets?
* What permissions are needed to create and manage the asset?
* Kibana Assets
* Is it a saved object and if yes, what is it's visibiliy? (space agnostic, shared, ...)
* Is there anything related to the indexing strategy?
* What permissions are needed to create and manage the asset?
* Kibana Ingest Manager
* What are the CRUD operations for the asset?
* Any special behaviour on upgrade?
* Under which name is the asset created / stored?

I filed this in the package-spec repo as it seems to be the central place where all the parts come together.