IWebXRHitTestOptions should expose options to hit test against XRHitTestTrackableType instead of assume a default implementation

[hendryems]

Is your feature request related to a problem? Please describe.
When hit testing against real world geometry (e.g. through ARKit/ARCore), the hit test seems to be very specific to a particular implementation.

E.g. On the ARCore implementation [Source link], this is XRHitTestTrackableType.mesh which seems to be the most restrictive.

            // Iterate over the results and pull out only those that match the desired TrackableType.  For now we are limiting results to
            // just hits against the Plane, and further scoping that to Poses that are contained in the polygon of the detected mesh.
            // This is equivalent to XRHitTestTrackableType.mesh (https://immersive-web.github.io/hit-test/#hit-test-trackable-type-enum).

For apps looking to build an app using BabylonJS as its XR abstraction, it is desirable to be able to configure the specifics of what hit testing would be against.

Describe the solution you'd like
When configurating a WebXRHitTest, a XRHitTestTrackableType option (through IWebXRHitTestOptions ?) should be exposed.

Discussion
BabylonJS/BabylonReactNative#53

[RaananW]

Sure! makes sense. I have already added entityType to the webxr.d.ts file, it is a matter of exposing it in the configuration. Also noticed the mesh type is missing, will add that as well