feat: Make interactive steps only accessible to specific user

[PhMemmel]

A use case that came up for us:

We need to trigger interactive steps, but we only want to show the steps to specific users/roles.

Implementation idea: Add a method is_available_to_user($currentuser, $course, ...) to the interactionlibbase class which returns true by default but can be overwritten by a step subplugin to avoid showing this step to the current user. This method has to be respected by the table view of the courses.
Another idea would be to implement a method generating an array of user ids which are allowed to interact with this step. Not 100% sure, what's the better approach.

I realize that this is somewhat similar to the get_relevant_capability method, but unfortunately, defining a capability is not enough for our use case.

I talked to some other devs about this and we came to the conclusion that this would be an easy yet effective approach. I'm going to implement this and will send a PR. If you have thoughts on the idea, the mentioned approach or the implementation details, let me know!