Make GazeProvider a proper controller

[FejZa]

Overview

The gaze provider has always been a bit of a weird curiosity in the toolkit. It's time to mature it into a proper controller that makes it more versatile and usable.

Proposed changes

• Introduce controller type "GazeController" operated by a default "Gaze Controller Service Module" of the input service
• The GazeController operates and behaves just as before but is treated as a proper controller by the input service that can easily be configued or disabled via the service module

Benefits

• Follows proper architecture and design principles of the toolkit
• Sets the foundation for special Gaze Controllers, such as an "Eye Tracking Gaze Controller"

[SimonDarksideJ]

Another thing to factor in this upgrade, is due to the GazeProvider currently causing a Null ref exception onDisable due to the Input Service being unregistered.
As a module, it would not have this issue.