cache: Re-implement cache on top of our external storage

The goal is to reduce egress on FDO server. In this patch, the cache is split from fetch.py into it's own set of commands in cache.py. It now implements fetch-cache, gen-cache and upload-chache. upload-cache can only work if the SSH private key needed to upload has been set into CERBERO_PRIVATE_SSH_KEY environment or is available in your .ssh folder. The key will be made availabe through configuration in gstreamer/cerbero CI configuration and only available to protected branches on the gstreamer namespace.