Add feature for optional request compression (#8767)

Adds a new feature
`enable_request_compression` that will compress using zstd requests to
the codex-backend. Currently only enabled for codex-backend so only enabled for openai providers when using chatgpt::auth even when the feature is enabled

Added a new info log line too for evaluating the compression ratio and
overhead off compressing before requesting. You can enable with
`RUST_LOG=$RUST_LOG,codex_client::transport=info`

```
2026-01-06T00:09:48.272113Z  INFO codex_client::transport: Compressed request body with zstd pre_compression_bytes=28914 post_compression_bytes=11485 compression_duration_ms=0
```

codex-rs/codex-client/src/default_client.rs

@@ -104,6 +104,13 @@ impl CodexRequestBuilder {
         self.map(|builder| builder.json(value))
     }
 
+    pub fn body<B>(self, body: B) -> Self
+    where
+        B: Into<reqwest::Body>,
+    {
+        self.map(|builder| builder.body(body))
+    }
+
     pub async fn send(self) -> Result<Response, reqwest::Error> {
         let headers = trace_headers();