Fixes MRTK3's Tap to Place solver to work with any hand and interactor

[AMollis]

Overview

Fixes MRTK3's Tap to Place solver to work with any hand and any interactor (even speech). This change, on StartPlacement, queries the XRInteractionManager for all registered interactors, and then registers for the interactors' select events. This is an alternative to requiring the developer to specify particular actions.

Note, this change also removes the StatefulInteractable requirement. The consumer of TapToPlace is now required to determine when StartPlacement is invoked. This change is meant to provide extensibility for future scenarios which may not require or use StatefulInteractables.

This also adds Unit Tests to validate TapToPlace.

Changes

• Fixes: The MRTK3 Tap to Place solver only works with the right hand #11527

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