S1AP: Add an S1AP context to be able to determine which message and
message type an IE was sent in. Needed to dissect proprietarry data.

Change-Id: Ie75a2f6a544cb33e22c42457b0edd83e6456bfe5
Reviewed-on: https://code.wireshark.org/review/4910
Reviewed-by: Anders Broman <a.broman58@gmail.com>