gddo-server: add isRobot field to gddoEvent

This will indicate whether the request to pkg.go.dev was made by a
robot.

Change-Id: I64931b9ff8a4da575d0614771a0575f2bd0746a0
Reviewed-on: https://go-review.googlesource.com/c/gddo/+/235448
Reviewed-by: Jonathan Amsterdam <jba@google.com>
Reviewed-by: Andrew Bonventre <andybons@golang.org>
Reviewed-by: Julie Qiu <julie@golang.org>
Run-TryBot: Jonathan Amsterdam <jba@google.com>