feat: introduce ExternalSandbox policy (#8290)

## Description

Introduced `ExternalSandbox` policy to cover use case when sandbox
defined by outside environment, effectively it translates to
`SandboxMode#DangerFullAccess` for file system (since sandbox configured
on container level) and configurable `network_access` (either Restricted
or Enabled by outside environment).

as example you can configure `ExternalSandbox` policy as part of
`sendUserTurn` v1 app_server API:

```
 {
            "conversationId": <id>,
            "cwd": <cwd>,
            "approvalPolicy": "never",
            "sandboxPolicy": {
                  "type": ""external-sandbox",
                  "network_access": "enabled"/"restricted"
            },
            "model": <model>,
            "effort": <effort>,
            ....
        }
```

codex-rs/windows-sandbox-rs/src/audit.rs

@@ -271,7 +271,7 @@ pub fn apply_capability_denies_for_world_writable(
             })?,
             Vec::new(),
         ),
-        SandboxPolicy::DangerFullAccess => {
+        SandboxPolicy::DangerFullAccess | SandboxPolicy::ExternalSandbox { .. } => {
             return Ok(());
         }
     };