Refactor spec to be subscription based

[kearwood]

Feedback from the W3C F2F meeting and feedback on the initial PR is supporting the pattern of "subscribing" to values that change from frame to frame, rather than resolving a promise each frame. We should refactor XRFrame.getGlobalLightEstimate() and XRFrame.getGlobalReflectionProbe() to be subscription based. This should be cohesive with the subscription pattern proposed for XR hit testing.

[kearwood]

If we are to activate the global ambient estimation via a feature policy flag, that could suffice to be an indication to the UA that the XRSession is subscribing to light estimation updates. If we are later supporting local light estimation, the subscription for each probe would be instantiated after XRSession creation.

[kearwood]

Perhaps we could proceed by describing this module as "Global Lighting Estimation" and, if there is enough interest, later define a "Local Lighting Estimation" module which builds on this with a subscription-based request for lighting localized around a hit test location.