Define how the application can unsubscribe from hit test / remove hit test source

[bialpio]

Currently, the explainer does not specify how the application could unsubscribe from the hit test.

I think we have a few possible solutions here:

• rely on the GC to clean up the objects - if the application is no longer interested in hit test results, it can simply drop its references to XRHitTestSource
• provide an explicit method on XRHitTestSource to signal that the application is no longer interested in obtaining the results - hit test source will be active until that happens, even if the application drops all references to its XRHitTestSource
• provide an explicit method so that the app does not have to rely on GC, and also rely on the GC if the app drops all references to its XRHitTestSource