Add support for proxy semver 0.2.4 API

Cargo.toml

@@ -24,3 +24,7 @@ tracing = "0.1"
 
 [lib]
 path = "src/imageproxy.rs"
+
+[features]
+# See https://github.com/containers/skopeo/blob/03da797e42374892bca8759668adb0b06d087876/cmd/skopeo/proxy.go#L95
+proxy_v0_2_4 = []

src/imageproxy.rs

@@ -333,6 +333,20 @@ impl ImageProxy {
         Ok(OpenedImage(imgid))
     }
 
+    #[instrument]
+    #[cfg(feature = "proxy_v0_2_4")]
+    pub async fn open_image_optional(&self, imgref: &str) -> Result<Option<OpenedImage>> {
+        tracing::debug!("opening image");
+        let (imgid, _) = self
+            .impl_request::<u32, _, _>("OpenImageOptional", [imgref])
+            .await?;
+        if imgid == 0 {
+            Ok(None)
+        } else {
+            Ok(Some(OpenedImage(imgid)))
+        }
+    }
+
     #[instrument]
     pub async fn close_image(&self, img: &OpenedImage) -> Result<()> {
         tracing::debug!("closing image");