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-auth/Cargo.toml

@@ -11,6 +11,7 @@ once_cell = { workspace = true }
 reqwest = { workspace = true }
 reqwest-middleware = { workspace = true }
 rust-netrc = { workspace = true }
+serde = { workspace = true, optional = true }
 thiserror = { workspace = true }
 tracing = { workspace = true }
 url = { workspace = true }