usb: samples: Add Extension descriptor to webUSB sample.

WebUSB sample is using BOS descriptor. Because of that
the bcdUSB field of device descriptor is set to 0x0210.
This requires for the BOS descriptor to have LPM support.
LPM support use additional descriptor that the HOST can
read by requesting BOS desc. The descriptor is called
Extension descriptor and is specified in `USB Link Power
Management ECN` document considered a part of USB 2.0
spec.

This patch adds missing part of the BOS descriptor and
fixes issue with webUSB sample not passing i'LPM L1 Suspend
Resume Test' from USB3CV test tool.

Signed-off-by: Emil Obalski <emil.obalski@nordicsemi.no>