Matrix/Parallelization for custom (composite) Actions

[mmailaender]

It would be great to have inside custom (composite) actions Matrix available, like for workflows (Or some other parallelization feature).

Why? We're building currently a composite action that we want to share on the marketplace. This action helps you to do easy IaC for Fauna databases. For that we build, test and deploy the db resources to the database. But if you run the tests (Integration tests), it can take a high amount of time (In our case already an hour). With parallelization, we could reduce the time drastically.

And because the test part is wrapped between the build & deploy step, we can't do it in a workflow (The tests are running against an ephemeral database). We would lose the benefit of having a complete capsuled action, that we can share with others and that is really easy to use and fast.