Add python-downloads-json-url option for uv.toml to configure custom Python installations via JSON URL (#12974)

## Summary

Part of #12838. Allow users to configure `python-downloads-json-url` in
`uv.toml` and not just from env.

I followed similar PR #8695, so same as there it's also available in the
CLI (I think maybe it's better not to be configurable from the CLI, but
since the mirror parameters are, I think it's better to do the same)


## Test Plan

<!-- How was it tested? -->

crates/uv-python/src/downloads.rs

@@ -268,8 +268,9 @@ impl PythonDownloadRequest {
     /// Iterate over all [`PythonDownload`]'s that match this request.
     pub fn iter_downloads(
         &self,
+        python_downloads_json_url: Option<&str>,
     ) -> Result<impl Iterator<Item = &'static ManagedPythonDownload> + use<'_>, Error> {
-        Ok(ManagedPythonDownload::iter_all()?
+        Ok(ManagedPythonDownload::iter_all(python_downloads_json_url)?
             .filter(move |download| self.satisfied_by_download(download)))
     }
 
@@ -496,8 +497,9 @@ impl ManagedPythonDownload {
     /// be searched for — even if a pre-release was not explicitly requested.
     pub fn from_request(
         request: &PythonDownloadRequest,
+        python_downloads_json_url: Option<&str>,
     ) -> Result<&'static ManagedPythonDownload, Error> {
-        if let Some(download) = request.iter_downloads()?.next() {
+        if let Some(download) = request.iter_downloads(python_downloads_json_url)?.next() {
             return Ok(download);
         }
 
@@ -505,7 +507,7 @@ impl ManagedPythonDownload {
             if let Some(download) = request
                 .clone()
                 .with_prereleases(true)
-                .iter_downloads()?
+                .iter_downloads(python_downloads_json_url)?
                 .next()
             {
                 return Ok(download);
@@ -514,32 +516,36 @@ impl ManagedPythonDownload {
 
         Err(Error::NoDownloadFound(request.clone()))
     }
-
     //noinspection RsUnresolvedPath - RustRover can't see through the `include!`
+
     /// Iterate over all [`ManagedPythonDownload`]s.
-    pub fn iter_all() -> Result<impl Iterator<Item = &'static ManagedPythonDownload>, Error> {
-        let runtime_source = std::env::var(EnvVars::UV_PYTHON_DOWNLOADS_JSON_URL);
-
+    ///
+    /// Note: The list is generated on the first call to this function.
+    /// so `python_downloads_json_url` is only used in the first call to this function.
+    pub fn iter_all(
+        python_downloads_json_url: Option<&str>,
+    ) -> Result<impl Iterator<Item = &'static ManagedPythonDownload>, Error> {
         let downloads = PYTHON_DOWNLOADS.get_or_try_init(|| {
-            let json_downloads: HashMap<String, JsonPythonDownload> =
-                if let Ok(json_source) = &runtime_source {
-                    if Url::parse(json_source).is_ok() {
-                        return Err(Error::RemoteJSONNotSupported());
-                    }
+            let json_downloads: HashMap<String, JsonPythonDownload> = if let Some(json_source) =
+                python_downloads_json_url
+            {
+                if Url::parse(json_source).is_ok() {
+                    return Err(Error::RemoteJSONNotSupported());
+                }
 
-                    let file = match fs_err::File::open(json_source) {
-                        Ok(file) => file,
-                        Err(e) => { Err(Error::Io(e)) }?,
-                    };
-
-                    serde_json::from_reader(file)
-                        .map_err(|e| Error::InvalidPythonDownloadsJSON(json_source.clone(), e))?
-                } else {
-                    serde_json::from_str(BUILTIN_PYTHON_DOWNLOADS_JSON).map_err(|e| {
-                        Error::InvalidPythonDownloadsJSON("EMBEDDED IN THE BINARY".to_string(), e)
-                    })?
+                let file = match fs_err::File::open(json_source) {
+                    Ok(file) => file,
+                    Err(e) => { Err(Error::Io(e)) }?,
                 };
 
+                serde_json::from_reader(file)
+                    .map_err(|e| Error::InvalidPythonDownloadsJSON(json_source.to_string(), e))?
+            } else {
+                serde_json::from_str(BUILTIN_PYTHON_DOWNLOADS_JSON).map_err(|e| {
+                    Error::InvalidPythonDownloadsJSON("EMBEDDED IN THE BINARY".to_string(), e)
+                })?
+            };
+
             let result = parse_json_downloads(json_downloads);
             Ok(Cow::Owned(result))
         })?;

crates/uv-python/src/installation.rs

@@ -88,6 +88,7 @@ impl PythonInstallation {
         reporter: Option<&dyn Reporter>,
         python_install_mirror: Option<&str>,
         pypy_install_mirror: Option<&str>,
+        python_downloads_json_url: Option<&str>,
     ) -> Result<Self, Error> {
         let request = request.unwrap_or(&PythonRequest::Default);
 
@@ -127,6 +128,7 @@ impl PythonInstallation {
             reporter,
             python_install_mirror,
             pypy_install_mirror,
+            python_downloads_json_url,
         )
         .await
         {
@@ -146,13 +148,14 @@ impl PythonInstallation {
         reporter: Option<&dyn Reporter>,
         python_install_mirror: Option<&str>,
         pypy_install_mirror: Option<&str>,
+        python_downloads_json_url: Option<&str>,
     ) -> Result<Self, Error> {
         let installations = ManagedPythonInstallations::from_settings(None)?.init()?;
         let installations_dir = installations.root();
         let scratch_dir = installations.scratch();
         let _lock = installations.lock().await?;
 
-        let download = ManagedPythonDownload::from_request(&request)?;
+        let download = ManagedPythonDownload::from_request(&request, python_downloads_json_url)?;
         let client = client_builder.build();
 
         info!("Fetching requested Python...");