Adding gaze divergence module

[dunity]

Purpose of this PR

Add functionality to any implementer, allowing them to detect when the HMD is focused upon a given transform, within a defined (degree-based)range.

The "foot-indicator-smooth-opacity" branch utilizes this module, in order to fade the PlayerFloor UI contents in/out, according to when they're being focused upon.

Testing status

Tested in various scenes & conditions. I haven't encountered issues with the addition of this Module.

Technical risk

Low. Outside of the addition of the module registration in EditorVR.cs, there's not much room for this code to cause conflicts.

Comments to reviewers

The module's functionality can be tested with the "foot-indicator-smooth-opacity" branch (still in progress).

[StephenHodgson]

Please make sure this works with rect transforms too, thanks 😀

[dunity]

• Addressed @AndrewTHEManeri feedback
• Added summary to gazeStable property & class
• Cached the m_GazeSourceTransform's rotation in update