Purge cache by tag

[tc80]

Addresses #178 .

The bot will purge our CF zone's cache using tags.

• If a new package is found, purge /packages tag (contains list of all pkgs).
• If new versions were added or a package's metadata was updated, purge <pkg name> tag (package-level metadata, list of versions, aggregated metadata)

We need to be careful of race conditions.

• If cache is purged but KV values are not yet propagated globally, it is possible the Worker will fetch stale entries from KV and cache them.
• Maybe we can set Worker's max-age to a day instead of a year (if we implement this)

If we do this change, we will need to update bot-ansible:

• WORKERS_KV_ACCOUNT_ID --> CF_ACCOUNT_ID
• WORKERS_KV_API_TOKEN --> CF_API_TOKEN
• need to add CF_ZONE_ID
• need to update privileges of CF_API_TOKEN to be able to purge zone's cache