Support volumes for Lambda

[r4victor]

Volumes can be supported via File Systems. File Systems are persistent disks that can be mounted at fixed mount points of one or more VMs. It’s not possible to use a File System for root disk. File Systems are tied to regions. It’s not possible to access File System contents without attaching to it to a VM. The data may not be available if there is no capacity. File Systems have no size/limits (8 exabytes). They are mounted as Virtiofs. Pricing is $0.20/GB/month.

Since File Systems have no size/limits, it may not be possible to support volumes for Lambda on dstack Sky where we need to account for user spendings.