Onboard test-infra buckets to notification-based updates for TestGrid

[michelle192837]

TestGrid config (in GCP/testgrid, e.g. https://github.com/GoogleCloudPlatform/testgrid/blob/main/cluster/prod/updater.yaml#L46) allows listing buckets to subscribe to notifications for. When objects update in those buckets, TestGrid can queue up the related group(s) to update, instead of purely relying on a poll-based update cycle.

There's subscriptions for a few buckets at the moment, but especially in light of changing the main logs bucket for test-infra (#33381), I should add additional buckets to the flags in updater.

(Later these should be automatically determined in the Updater, but we need to handle enabling notifications + permissions first).

[michelle192837]

/sig testing