[RFC] Add extensions field to CreateSandboxRequest for custom behaviors

[ninan-nn]

Why do you need it?

Currently, CreateSandboxRequest lacks a generic way to handle provider-specific features or experimental parameters. This leads to frequent and unnecessary changes to the core API surface.

We propose adding an extensions field to the schema to:

• Enable Extensibility: Support internal-only or experimental parameters (e.g., specialized scheduling hints) without schema updates.

• Ensure SDK Stability: Allow new features to be passed through by SDKs transparently before they are promoted to core fields.

• Decouple Concerns: Keep the core API clean while allowing flexible, server-side customized behaviors.

How could it be?

Add the following field to CreateSandboxRequest in the OpenAPI spec:

extensions:
      type: object
      additionalProperties: true
      description: |
        Optional key-value pairs for server-side customized behaviors and experimental features.
        
        **Best Practice**: 
        - Use agreed-upon keys to avoid collisions.
        - SDKs should pass this object through transparently.