Address #2220 (slow download perf against PyPi mirror) (#2319)

## Summary

Addressing the extremely slow performance detailed in
https://github.com/astral-sh/uv/issues/2220. There are two changes to
increase download performance:

1. setting `accept-encoding: identity`, in the spirit of
https://github.com/pypa/pip/pull/1688
2. increasing buffer from 8KiB to 128KiB. 

### 1. accept-encoding: identity

I think this related `pip` PR has a good explanation of what's going on:
https://github.com/pypa/pip/pull/1688

```
  # We use Accept-Encoding: identity here because requests
  # defaults to accepting compressed responses. This breaks in
  # a variety of ways depending on how the server is configured.
  # - Some servers will notice that the file isn't a compressible
  #   file and will leave the file alone and with an empty
  #   Content-Encoding
  # - Some servers will notice that the file is already
  #   compressed and will leave the file alone and will add a
  #   Content-Encoding: gzip header
  # - Some servers won't notice anything at all and will take
  #   a file that's already been compressed and compress it again
  #   and set the Content-Encoding: gzip header
```

The `files.pythonhosted.org` server is the 1st kind. Example debug log I
added in `uv` when installing against PyPI:

<img width="1459" alt="image"
src="ef10d758-46aa-4c8e-9dba-47f33437401b">

(there is no `content-encoding` header in this response, the `whl`
hasn't been compressed, and there is a content-length header)

Our internal mirror is the third case. It does seem sensible that our
mirror should be modified to act like the 1st kind. But `uv` should
handle all three cases like `pip` does.

### 2. buffer increase

In https://github.com/astral-sh/uv/issues/2220 I observed that `pip`'s
downloading was causing up-to 128KiB flushes in our mirror.

After fix 1, `uv` was still only causing up-to 8KiB flushes, and was
slower to download than `pip`. Increasing this buffer from the default
8KiB led to a download performance improvement against our mirror and
the expected observed 128KiB flushes.

## Test Plan

Ran benchmarking as instructed by @charliermarsh 

<img width="1447" alt="image"
src="840d9c8d-4b98-4bfa-89f3-073a2dec1f23">

No performance improvement or regression.

crates/uv-distribution/src/distribution_database.rs

@@ -180,7 +180,19 @@ impl<'a, Context: BuildContext + Send + Sync> DistributionDatabase<'a, Context>
                     .instrument(info_span!("download", wheel = %wheel))
                 };
 
-                let req = self.client.cached_client().uncached().get(url).build()?;
+                let req = self
+                    .client
+                    .cached_client()
+                    .uncached()
+                    .get(url)
+                    .header(
+                        // `reqwest` defaults to accepting compressed responses.
+                        // Specify identity encoding to get consistent .whl downloading
+                        // behavior from servers. ref: https://github.com/pypa/pip/pull/1688
+                        "accept-encoding",
+                        reqwest::header::HeaderValue::from_static("identity"),
+                    )
+                    .build()?;
                 let cache_control = match self.client.connectivity() {
                     Connectivity::Online => CacheControl::from(
                         self.cache

crates/uv-extract/src/stream.rs

@@ -18,7 +18,7 @@ pub async fn unzip<R: tokio::io::AsyncRead + Unpin>(
     target: impl AsRef<Path>,
 ) -> Result<(), Error> {
     let target = target.as_ref();
-    let mut reader = reader.compat();
+    let mut reader = futures::io::BufReader::with_capacity(128 * 1024, reader.compat());
     let mut zip = async_zip::base::read::stream::ZipFileReader::new(&mut reader);
 
     let mut directories = FxHashSet::default();