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/request.rs

@@ -5,12 +5,20 @@ use serde::Serialize;
 use serde_json::Value;
 use std::time::Duration;
 
+#[derive(Debug, Clone, Copy, Default, PartialEq, Eq)]
+pub enum RequestCompression {
+    #[default]
+    None,
+    Zstd,
+}
+
 #[derive(Debug, Clone)]
 pub struct Request {
     pub method: Method,
     pub url: String,
     pub headers: HeaderMap,
     pub body: Option<Value>,
+    pub compression: RequestCompression,
     pub timeout: Option<Duration>,
 }
 
@@ -21,6 +29,7 @@ impl Request {
             url,
             headers: HeaderMap::new(),
             body: None,
+            compression: RequestCompression::None,
             timeout: None,
         }
     }
@@ -29,6 +38,11 @@ impl Request {
         self.body = serde_json::to_value(body).ok();
         self
     }
+
+    pub fn with_compression(mut self, compression: RequestCompression) -> Self {
+        self.compression = compression;
+        self
+    }
 }
 
 #[derive(Debug, Clone)]