prevent unused time import in event streams

[lucix-aws]

Event stream codegen blindly imports "time", but doesn't actually use it unless one of the following is true:

• there's a stream with input
• a payload or header used in event streaming has a timestamp field

This patch adds a compile-time reference to prevent vet from failing on the unused import (as we've done with "awserr" whose usage is also conditional in event streams).