Add Daytona backend

[peterschmidt85]

This PR adds Daytona backend support for CPU/GPU runs and volumes. It uses the online Daytona provider added in dstackai/gpuhunt#263.

• Docker images: Uses Daytona's Declarative Builder to build images on demand, instead of registering pre-built Snapshots. The Dockerfile starts with FROM <configured image> and overrides the user and startup command so dstack can install and start the runner as root. Daytona's user option does not override the image's USER.
• Private registries: Daytona requires a Registry containing the registry URL and credentials before it can pull a private image. When registry_auth is set, the backend creates this Registry and deletes it after deleting the sandbox.
• Volumes: Daytona Volumes are S3-backed FUSE mounts. The API has no size or region parameters; dstack accepts a volume configuration without these fields and lets runs share the volume across Daytona regions.

• Release gpuhunt 0.1.31.
• Replace the main dependency with gpuhunt==0.1.31 and remove allow-direct-references.

[peterschmidt85]

@mislavivanda would you like to review it?

[mislavivanda]

@mislavivanda would you like to review it?

Hey @peterschmidt85, sorry for a small delay here.

I've ran the backend end-to-end against production Daytona from this branch: CPU and GPU task both provisioned, ran and exited 0 - offers, quota labels, Declarative Builder path and the Toolbox runner bootstrap all behave exactly as the platform intends. LGTM.

One rough edge found by testing with a minimally-scoped API key: offers require the read:limits permission (organizations/{id}/usage returns 403 without it) and termination requires delete:sandboxes - but an under-scoped key configures successfully and then either yields silently empty offers or loops in NotYetTerminated on teardown. Conveniently, api-keys/current - which validate_config already calls - returns the key's permissions array, so the fix is free: check the required scopes (write:sandboxes, delete:sandboxes, read:limits, plus registry/volume scopes when used) against that list and fail fast with the missing ones named. Worth also listing the minimum permissions in backends.md. Not blocking - green light from our side.