Support for pulling from multiple artifact caches

I'd like to provide multiple caches internally at my enterprise:

1. A permanent archive of builds for production.
2. A monthly rolling cache of automated builds for branches.
3. A size-limited LRU cache of developer-built (i.e. from laptop) artifacts.

For the BuildStream part of this, I'd like to be able to pull from all of these automatically, ideally in order of preference.

Some aims it meets:

• Seperate trusted and less-trusted pushers.
• Permanent retention of the most important artifacts.
• 'We only build once' for developers, even for newest shared sources.
• Flexibility for storage use strategy.