Add support for global uv python pin (#12115)

These changes add support for

```
uv python pin 3.12 --global 
```

This adds the specified version to a `.python-version` file in the
user-level config directory. uv will now use the user-level version as a
fallback if no version is found in the project directory or its
ancestors.

Closes #4972

crates/uv-python/src/version_files.rs

@@ -4,6 +4,7 @@ use std::path::{Path, PathBuf};
 use fs_err as fs;
 use itertools::Itertools;
 use tracing::debug;
+use uv_dirs::user_uv_config_dir;
 use uv_fs::Simplified;
 
 use crate::PythonRequest;
@@ -69,7 +70,13 @@ impl PythonVersionFile {
         options: &DiscoveryOptions<'_>,
     ) -> Result<Option<Self>, std::io::Error> {
         let Some(path) = Self::find_nearest(working_directory, options) else {
-            return Ok(None);
+            // Not found in directory or its ancestors. Looking in user-level config.
+            return Ok(match user_uv_config_dir() {
+                Some(user_dir) => Self::discover_user_config(user_dir, options)
+                    .await?
+                    .or(None),
+                None => None,
+            });
         };
 
         if options.no_config {
@@ -84,6 +91,22 @@ impl PythonVersionFile {
         Self::try_from_path(path).await
     }
 
+    pub async fn discover_user_config(
+        user_config_working_directory: impl AsRef<Path>,
+        options: &DiscoveryOptions<'_>,
+    ) -> Result<Option<Self>, std::io::Error> {
+        if !options.no_config {
+            if let Some(path) =
+                Self::find_in_directory(user_config_working_directory.as_ref(), options)
+                    .into_iter()
+                    .find(|path| path.is_file())
+            {
+                return Self::try_from_path(path).await;
+            }
+        }
+        Ok(None)
+    }
+
     fn find_nearest(path: impl AsRef<Path>, options: &DiscoveryOptions<'_>) -> Option<PathBuf> {
         path.as_ref()
             .ancestors()