[Meta] Improve `kubernetes` backend

[un-def]

Essential:

• [x] Request resources according to the dstack configuration
• [x] Multi-node support (distributed tasks running on fleets with cluster placement)

Strategic:

• [x] AMD GPUs support
• [ ] Allow to configure multiple clusters per backend (e.g. per region)
• [ ] Auto-scaling support (ideally, find a way to support it for any clouds)

Improvements:

• [x] Update the jump pod: use a lightweight image, restrict SSH access (see TODOs in _create_jump_pod_service)
• [x] Test and update (if required) the gateway functionality on managed/self-hosted Kubernetes other than EKS (see TODO in KubernetesCompute.create_gateway)