Redact Git credentials from pyproject.toml (#6074)

## Summary

We retain them if you use `--raw-sources`, but otherwise they're
removed. We still respect them in the subsequent `uv.lock` via an
in-process store.

Closes #6056.

crates/uv-git/src/credentials.rs

@@ -0,0 +1,21 @@
+use std::collections::HashMap;
+use std::sync::{Arc, RwLock};
+
+use cache_key::RepositoryUrl;
+use uv_auth::Credentials;
+
+/// A store for Git credentials.
+#[derive(Debug, Default)]
+pub struct GitStore(RwLock<HashMap<RepositoryUrl, Arc<Credentials>>>);
+
+impl GitStore {
+    /// Insert [`Credentials`] for the given URL into the store.
+    pub fn insert(&self, url: RepositoryUrl, credentials: Credentials) -> Option<Arc<Credentials>> {
+        self.0.write().unwrap().insert(url, Arc::new(credentials))
+    }
+
+    /// Get the [`Credentials`] for the given URL, if they exist.
+    pub fn get(&self, url: &RepositoryUrl) -> Option<Arc<Credentials>> {
+        self.0.read().unwrap().get(url).cloned()
+    }
+}