docs(rfc): add sandbox resource requirements proposal

[elezar]

Summary

Add RFC 0004 proposing a typed sandbox resource requirements model for CPU, memory, GPUs, and future resource domains. The RFC separates portable resource requirements from driver/platform-specific configuration and realization, and includes concrete realization examples for Kubernetes, Docker, Podman, and VM drivers.

Related Issue

Related to #1338 and #1340.

Changes

• Adds rfc/0004-sandbox-resource-requirements/README.md.
• Proposes SandboxSpec.resource_requirements with compute, device, dataset, and extension domains.
• Reserves JSON-formatted CLI input for --driver-config-json, mapped to SandboxTemplate.driver_config.
• Explicitly avoids exposing JSON-formatted portable resource request flags.
• Documents how CPU/memory and GPU requests map to Kubernetes resources, CDI device injection, and VM device assignment.
• Captures conflict handling between portable resource requirements and SandboxTemplate.resources passthrough.

Testing

• mise run pre-commit passes
• Unit tests added/updated (not applicable; RFC-only change)
• E2E tests added/updated (not applicable; RFC-only change)

Checklist

• Follows Conventional Commits
• Commits are signed off (DCO)