[Keep] Support multiple storage tiers
|Target version:||Arvados Future Sprints|
|Velocity based estimate||-|
As an Arvados system administrator, I'd like the ability take advantage of the cool/cold storage tiers offered by cloud vendors, in addition to the standard "hot" storage.
This would potentially require the ability to designate storage tier in some way (collection, all collections in a project, etc) as well as a way to migrate between hot and cool storage.