GCS backed tracked package list

The current deployment of pypi RSS uses a benchmark file to describe the tracked package set. This is strange because the benchmark file includes pinned versions. It also makes it harder to update the tracked packages set while continuing to run the RSS subscriber.

We should place out tracked package configuration in a GCS bucket that can be fetched and cached by release subscribers.

Locate the pypi RSS deployment, benchmark file, and release subscriber entry points to understand how the tracked package set is currently loaded. Then trace the configuration fetch and caching path; done means subscribers can use a GCS-hosted package set without relying on pinned versions in the benchmark file.

Written by the indexing model from the issue text.