Return computed hashes from metadata requests (#2951)

## Summary

This PR modifies the distribution database to return both the
`Metadata23` and the computed hashes when clients request metadata.

No behavior changes, but this will be necessary to power
`--generate-hashes`.

crates/uv-distribution/src/lib.rs

@@ -4,6 +4,7 @@ pub use download::LocalWheel;
 pub use error::Error;
 pub use git::{is_same_reference, to_precise};
 pub use index::{BuiltWheelIndex, RegistryWheelIndex};
+use pypi_types::{HashDigest, Metadata23};
 pub use reporter::Reporter;
 pub use source::SourceDistributionBuilder;
 
@@ -16,3 +17,21 @@ mod index;
 mod locks;
 mod reporter;
 mod source;
+
+/// The metadata associated with an archive.
+#[derive(Debug, Clone)]
+pub struct ArchiveMetadata {
+    /// The [`Metadata23`] for the underlying distribution.
+    pub metadata: Metadata23,
+    /// The hashes of the source or built archive.
+    pub hashes: Vec<HashDigest>,
+}
+
+impl From<Metadata23> for ArchiveMetadata {
+    fn from(metadata: Metadata23) -> Self {
+        Self {
+            metadata,
+            hashes: vec![],
+        }
+    }
+}