XR: Expose the OpenXR user paths in the event data for XR events

The use-case is to allow an event handler (in C or a plugin) to
distinguish which hand produced the XR event.

The alternative is to register separate actions for each hand (e.g.
"trigger_left" and "trigger_right"), and duplicate the device bindings
(Oculus, HTC Vive, etc) for each action. Other than the problem of code
duplication, this isn't conceptually efficient since "trigger_left" and
"trigger_right" both represent the same event "trigger", and the
identity of the hand that produced that event is just a property of
that event.

Adds two string fields to the XrEventData called user_path and
user_path_other. The user_path_other field will be populated if the
event is a bimanual one (i.e. two-handed). This follows the pattern
used by the rest of the XrEventData struct for bimanual events (e.g.
state, state_other).

Reviewed By: muxed-reality

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