[XR] Support controller as source of forward vector

[Corysia]

As discussed here, I would like to be able to allow my users to choose a controller as the source for the forward vector for smooth locomotion. This feature is supported in many games (e.g., Skyrim VR, Stormland, etc...) and is sometimes referred to as "wand forward" vs "HMD forward" which BabylonJS already supports. The advantage to this type of movement is that you can point your hand in the direction you wish to go, and look around at your surroundings without changing your trajectory. I believe most First Person Shooters are configured this way.

It's already possible to set either your left or right controller to handle movement and the opposite to handle rotation. Typically, the controller that is handling movement would be defined as the "forward" source. In the Unity XR Interaction Toolkit, there is an option to drop in an object that is used to determine the source for Vector.Foward and Vector.Up. Doing something similar would be ideal, such as adding a field for the feature manager.

            const smoothLocomotion = featureManager.enableFeature(WebXRFeatureName.MOVEMENT, 'latest', {
                xrInput: xr.input,
                floorMeshes: [this.scene.getMeshByName('ground1')!],
                customRegistrationConfigurations: leftHandConfiguration,
                // add options here
                movementOrientationFollowsViewerPose: false,
                // new option
                movementOrientationForwardSource: leftMotionController

where leftMotionController is a WebXRInputSource. Alternatively, it could be any object that has a matrix where a Vector.Forward, Vector.Right, and Vector.Up can be determined. This could offer more flexibility, allowing for the support of the camera following a vehicle such as an airplane.

A screenshot from the Unity XR Interaction Toolkit and their "Forward Source" field:

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