Allow registries to pre-provide core metadata (#15644)

## Summary

This PR adds support for the `application/vnd.pyx.simple.v1` content
type, similar to `application/vnd.pypi.simple.v1` with the exception
that it can also include core metadata for package-versions directly.

crates/uv-client/src/error.rs

@@ -75,6 +75,11 @@ impl Error {
         ErrorKind::BadHtml { source: err, url }.into()
     }
 
+    /// Create a new error from a `MessagePack` parsing error.
+    pub(crate) fn from_msgpack_err(err: rmp_serde::decode::Error, url: DisplaySafeUrl) -> Self {
+        ErrorKind::BadMessagePack { source: err, url }.into()
+    }
+
     /// Returns `true` if this error corresponds to an offline error.
     pub(crate) fn is_offline(&self) -> bool {
         matches!(&*self.kind, ErrorKind::Offline(_))
@@ -251,6 +256,12 @@ pub enum ErrorKind {
         url: DisplaySafeUrl,
     },
 
+    #[error("Received some unexpected MessagePack from {}", url)]
+    BadMessagePack {
+        source: rmp_serde::decode::Error,
+        url: DisplaySafeUrl,
+    },
+
     #[error("Failed to read zip with range requests: `{0}`")]
     AsyncHttpRangeReader(DisplaySafeUrl, #[source] AsyncHttpRangeReaderError),