OSEP-0011: Runtime Driver Plugin Interface

[AlexandrePh]

Summary

Proposes a formal RuntimeDriver interface for pluggable sandbox runtime backends.

Key design points:

• Multi-driver routing: A single lifecycle server loads multiple drivers simultaneously (e.g., Linux K8s + Windows Hyper-V). CreateSandboxRequest gains an optional driver field; omitting it uses the configured default
• External drivers via gRPC: Third-party backends (Windows, ACI, Wasm) run as separate processes and communicate via a standard gRPC protocol
• Capability discovery: Drivers declare supported operations at registration; GET /v1/drivers exposes loaded drivers and capabilities
• Zero breaking changes: Existing Docker/K8s backends become built-in drivers; all current configs continue to work

Motivation:

• Issue Support Windows-based sandbox runtime #438 requests Windows sandbox support — current architecture has no path without core changes
• OSEP-0007 (Fast Sandbox) already proposes a gRPC controller but as a one-off integration
• Adding each new backend to factory.py doesn't scale

Relationship to other OSEPs:

• Generalizes OSEP-0007 (Fast Sandbox) into a standard driver protocol
• Complements OSEP-0004 (Secure Runtime) — runtime class selection operates within a driver
• Enables future Windows, macOS (Apple Containerization), and cloud-managed backends

Looking for feedback on the multi-driver routing design and the gRPC external driver protocol before moving to provisional.

Closes #438 (partially — defines the interface; Windows driver is a separate effort)