[UX] Detect `pulling` job status for container-based backends

[peterschmidt85]

In container-based backends, we have to implement backend-specific logic to detect if the pod is assigned to an instance which means its successfully provisioned:

1. In runpod and vastai we can assume that if the pod is created, it's already assigned to a node. In runpod, there is machineId property in the pod and it's always filled if the pod is created, otherwise it's not created. In vastai the pod is bound to a particular instanceId which also means it's assigned from the very beginning.
2. In kubernetes, we could check if the pod is assigned to a node using the standard Kubernetes API.