non-IEnumerable interface for action masking

[chriselion]

Proposed change(s)

This fixes a few things that bugged me about IDiscreteActionMask:

• Removes the GetMask and ResetMask methods - these were part of the implementation in ActuatorDiscreteActionMask, but they weren't meant for public use (and there was no safe way to use them in an OnActionReceived callback).
• Adds a method that doesn't require an IEnumerable. The previous approach required creating an object to pass, and also needed to allocate memory to iterate over it.
• Made what you're doing more explicit, and also undoable - "WriteMask" always felt vague, and I always had to look up whether I was enabling or disabling the actions.

Things I haven't done yet but would like feedback on:

• Should we remove IDiscreteActionMask.WriteMask()? This would be a larger interface change for anyone using it, but I think it's for the best.
• Should we stop having this as an interface and use a concrete implementation instead? Making it an interface feels strange, as there's no way for a user to implement their own version and get it passed through the system (although this could change in the future if we want to decouple the implementations.

I'm leaning towards removing WriteMask but keeping the interface, but wanted to discuss it further.

I haven't updated existing tests or documentation yet; wanted to get feedback on the general approach first.

Useful links (Github issues, JIRA tickets, ML-Agents forum threads etc.)

https://jira.unity3d.com/browse/MLA-1831
https://jira.unity3d.com/browse/MLA-1728

Types of change(s)

• Code refactor
• Breaking change
• Documentation update
• Other (please describe)

Checklist

• Added tests that prove my fix is effective or that my feature works
• Updated the changelog (if applicable)
• Updated the documentation (if applicable)
• Updated the migration guide (if applicable)

Other comments

[chriselion]

This needs to be true