Make pack-objects cache configurable via TOML

Part of gitlab-com/gl-infra/scalability#953 (closed)

This change makes the pack-objects cache configurable via config.toml, as opposed to the hard-coded config values we have now.

When the cache is disabled in config.toml, which is the default, then PackObjectsHook uses a "null implementation" of the cache which does not use disk storage and treats every lookup as a cache miss.

We still keep the feature flag so if people want to use the cache they must enabled it in config.toml AND enable the feature flag.