Add btaggingLink in FTAG4

BinBin Dongrequested to merge
(removed):21.2-addbtaggingLink into 21.2

Add AuxDyn.btaggingLink for 201903 btagged VRTrack jets.

Tagging @vdao , @cschiavi , @sanmay and @duperrin to review.

Edited by BinBin Dong

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