The implementations aims to introduce shacache server/proxy on slapproxy. By updating slapos.cfg (or your software release), you can use a the local slapproxy to download software release binaries.
    
The API is fully compatible with libnetworkcache clients, so anything all API from ERP5 based shacache is present here.
    
The local shacache (on slapproxy) can be configured as proxy/cache, so whenever the cache is not present locally, it can connect to upstream to fetch the data, and save it before serve. This is important to "build" local cache to transfer to an offline-site.
    
An option was included on configure local, so the local shacache can be automatically configured.
    
This work is based on https://lab.nexedi.com/tsoulard/gakeshadeba/. 

Thanks to Titouan Soulard for the initial work that allow us build this extension to shacache.

/reviewed-on nexedi/slapos.core!913