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/common/Cargo.toml

@@ -21,3 +21,10 @@ toml = { workspace = true, optional = true }
 cli = ["clap", "serde", "toml"]
 elapsed = []
 sandbox_summary = []
+
+[dev-dependencies]
+clap = { workspace = true, features = ["derive", "wrap_help"] }
+codex-utils-absolute-path = { workspace = true }
+pretty_assertions = { workspace = true }
+serde = { workspace = true }
+toml = { workspace = true }