Update proto to match bindings of service changesv3.1

[lionel-nj]

This PR consists in a first attempt to merge the modifications from ServiceChanges v3.1 into the current .proto file for realtime GTFS feed.

See :

• GTFS-ServiceChanges vs extending GTFS-TripUpdates #113 (comment)
• proposal : https://docs.google.com/document/d/1oPfQ6Xvui0g3xiy1pNiyu5cWFLhvrTVyvYsMVKGyGWM/edit

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