E2B Protocol Compatibility: Metadata, TTL, and Snapshot/Fork APIs

**I'm using OpenKruise Agent with Substrate as the backend to implement sandbox management following the E2B protocol. The core APIs are already functional, but there are a few issues I'd like to raise. I understand the Substrate project's current priority is to get the end-to-end path working first, then iterate on more advanced capabilities. That said, I'd like to surface some questions early on and hear how the team is thinking about these.**

**1. Actor-level metadata and lifecycle fields:**
Support `Metadata.Annotations` and `Metadata.Labels` on Actors to enable user identity association. Additionally, support fields like `timeout` (TTL duration), `timeout_action` (expiry action: Pause / Delete), and `expires_at` (absolute expiry timestamp) to enable automatic suspension and termination based on TTL.

**2. Snapshot and fork interfaces:**
Support snapshot and fork APIs, with behavior aligned to the E2B protocol specification.

Related Issue in Substrate
#553 Support sandbox platforms running on Substrate