Add uv-workspace crate with settings discovery and deserialization (#3007)

## Summary

This PR adds basic struct definitions along with a "workspace" concept
for discovering settings. (The "workspace" terminology is used to match
Ruff; I did not invent it.)

A few notes:

- We discover any `pyproject.toml` or `uv.toml` file in any parent
directory of the current working directory. (We could adjust this to
look at the directories of the input files.)
- We don't actually do anything with the configuration yet; but those
PRs are large and I want this to be reviewed in isolation.

crates/uv-toolchain/src/python_version.rs

@@ -41,6 +41,14 @@ impl FromStr for PythonVersion {
     }
 }
 
+#[cfg(feature = "serde")]
+impl<'de> serde::Deserialize<'de> for PythonVersion {
+    fn deserialize<D: serde::Deserializer<'de>>(deserializer: D) -> Result<Self, D::Error> {
+        let s = String::deserialize(deserializer)?;
+        PythonVersion::from_str(&s).map_err(serde::de::Error::custom)
+    }
+}
+
 impl Display for PythonVersion {
     fn fmt(&self, f: &mut Formatter<'_>) -> std::fmt::Result {
         Display::fmt(&self.0, f)