[task scheduler] Add gen_tasks.go

This mimics the flow of infra/bots/recipes/swarm_trigger.py
but it builds the tasks.json file instead of triggering the
actual tasks.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2386663002

Review-Url: https://codereview.chromium.org/2386663002
2 files changed