Add Action / Alert assets

[ruflin]

Alerts / action will be coming in Kibana in the future: elastic/kibana#39093 The package format should support this.

[jlind23]

@mtojek Alerts/actions should be part of kibana tests rights? It should also be possible to export assets from elastic-package export command?

[mtojek]

I guess this is about modifying the package-spec like you did for Kibana tags.

[jsoriano]

Closing as duplicate of #121, let's keep discussions about this there.