Make pack-objects cache configurable

For our initial experiments with the Gitaly pack-objects cache we used a feature flag to turn the cache on and off, and all other configuration was hard-coded. This is not how this should work long term: configuration should be in Gitaly's configuration file, and it should be possible to change that section of the configuration file with Omnibus.

Gitaly is currently deployed with Omnibus, so we are not going to make the changes in the helm charts at this time.