Allow setting proxy variables via global / user configuration (#16918)
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## Summary
<!-- What's the purpose of the change? What does it do, and why? -->
This allows users to set the HTTP, HTTPS, and no proxy variables via the
configuration files like ~pyproject.toml~ and uv.toml.
Users can set like so:
`uv.toml`
```toml
https-proxy = "http://my_cool_proxy:10500"
http-proxy = "http://my_cool_proxy:10500"
no-proxy = [
"dontproxyme.com",
"localhost",
]
```
Resolves https://github.com/astral-sh/uv/issues/9472
## Test Plan
<!-- How was it tested? -->
It also adds a new integration test for the proxy support in
`uv-client`.
This was tested on some of our developer machines with our proxy setup
using `~/.config/uv/uv.toml` with values like:
```toml
https-proxy = "http://my_cool_proxy:10500"
http-proxy = "http://my_cool_proxy:10500"
no-proxy = [
"dontproxyme.com",
"localhost",
]
```
---------
Signed-off-by: Eli Uriegas <eliuriegas@meta.com>
Co-authored-by: Zanie Blue <contact@zanie.dev>