Introduce mobile impression tracking with Open Measurement

[ValentinPostindustria]

Original issue.

[patmmccann]

@justadreamer we're considering a release soon, have you guys arranged for review on this?

[YuriyVelichkoPI]

Hi @patmmccann ! it is a part of Prebid Mobile initiative where we introduced opt-in OpenMeasurement based burl tracking. It would be good if someone review and merge.

The introduced approach don't change the current behaviour until publisher don't add the follwing code to their creative.

ucTagData.mobileImpressionTracker = "open_measurement";

After that the Open Measurement framework will be responsible for tracking the burl event but not the PUC, and without dependency on MRAID.