Enable TLS native root toggling at runtime (#2362)
## Summary
It turns out that on macOS, reading the native certificates can add
hundreds of milliseconds to client initialization. This PR makes
`--native-tls` a command-line flag, to toggle (at runtime) the choice of
the `webpki` roots or the native system roots.
You can't accomplish this kind of configuration with the `reqwest`
builder API, so instead, I pulled out the heart of that logic from the
crate
(https://github.com/seanmonstar/reqwest/blob/e3192638518d577759dd89da489175b8f992b12f/src/async_impl/client.rs#L498),
and modified it to allow toggling a choice of root.
Note that there's an open PR for this in reqwest
(https://github.com/seanmonstar/reqwest/pull/1848), along with an issue
(https://github.com/seanmonstar/reqwest/issues/1843), which I may ping,
but it's been around for a while and I believe reqwest is focused on its
next major release.
Closes https://github.com/astral-sh/uv/issues/2346.
