[XRI3] Consider deprecating IPoseSource in favor of assignable tracked pose drivers

[keveleigh]

Describe the problem

IPoseSource was created as a data source for interactors in an XRController-world when interactors didn't align exactly with the device's pose (and there was a 1-to-1 mapping of device pose to XRController).

Describe the solution you'd like

With the move to assignable IXRInputButtonReaders in XRI3, it would be interesting to investigate if we can follow a similar pattern with tracked pose drivers in the scene mapping to specific poses like the poke pose, the aim pose, etc.