[Feature] Allow to attach multiple volumes based on backend and region

[peterschmidt85]

Problem:

For example, when I use RunPod, I never know in advance if the instance I will use will be available in a particular region of my configured volume.
Thus, if I want to persist data, there is almost no convenient way given the fact that I don't know the region in advance.

Solution:

Allow to create volumes in multiple backends and regions, and allow to specify a list of volumes so dstack can automatically use one of them. This is a game changer feature for use cases when multiple regions (or backends) are going to be used.

volumes:
  - name: [my-aws-eu-west-1-volume, my-aws-us-east-1-volume]
    path: /volume_data

[playerzer0x]

+1. Would love to see this feature implemented ASAP. I currently have to cancel and retry different GPU + volume configurations to find one that works.