Enable usage logs to track the downloads for artifacts in knative-releases project

Currently, the network egress cost for `knative-releases` project is ~$1k per month, and it's because this project hosts all the Knative release artifacts in GCS and GCR, and all the Knative users will download them from it.

As there are more and more Knative users, the cost for it will keep growing. To get a better understanding on where the costs come from and also for potential future improvements, it'll better track the usage for the Knative releases. Usage logs is the recommended solution to do this - https://cloud.google.com/storage/docs/access-logs

/cc @upodroid

Start by reviewing the knative-releases project’s GCS and GCR artifact hosting setup and the Google Cloud usage-logs guidance linked in the issue. Identify where logging can be enabled for release artifact access, then verify that downloads produce usable logs for cost and usage analysis.

Written by the indexing model from the issue text.