dissectors/llrp: Add support for Impinj's Hub extension

Based on Impinj extension schema v1.4.

Change-Id: Ica9493a4527ae800d6e5daaf2d86b8cfdf479ed1
Signed-off-by: Petr Štetiar <petr.stetiar@gaben.cz>
Reviewed-on: https://code.wireshark.org/review/2981
Reviewed-by: Alexis La Goutte <alexis.lagoutte@gmail.com>
Tested-by: Alexis La Goutte <alexis.lagoutte@gmail.com>
Reviewed-by: Evan Huus <eapache@gmail.com>