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-resolver/src/exclude_newer.rs

@@ -4,6 +4,7 @@ use chrono::{DateTime, Days, NaiveDate, NaiveTime, Utc};
 
 /// A timestamp that excludes files newer than it.
 #[derive(Debug, Copy, Clone)]
+#[cfg_attr(feature = "serde", derive(serde::Deserialize, serde::Serialize))]
 pub struct ExcludeNewer(DateTime<Utc>);
 
 impl ExcludeNewer {