Add GenericResult to sandbox tunnels response

[pawalt]

Describe your changes

This allows us to know if the SandboxGetTunnels RPC returned because of a timeout or full data.

Backward/forward compatibility checks

---

• Client+Server: this change is compatible with old servers
• Client forward compatibility: this change ensures client can accept data intended for later versions of itself

This is not a backwards-compatible proto change, but it's OK because there's no code (and there will be no code) that uses the old version.

---

Changelog

[pawalt]

@prbot approve

[pawalt]

@prbot automerge