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.