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>

crates/uv-dev/src/generate_options_reference.rs

@@ -283,26 +283,40 @@ fn emit_field(output: &mut String, name: &str, field: &OptionField, parents: &[S
             option_type: OptionType::Configuration,
             ..
         } => {
-            output.push_str(&format_tab(
-                "pyproject.toml",
-                &format_header(
-                    field.scope,
+            // For uv_toml_only options, only show the uv.toml tab
+            if field.uv_toml_only {
+                output.push_str(&format_code(
+                    "uv.toml",
+                    &format_header(
+                        field.scope,
+                        field.example,
+                        parents,
+                        ConfigurationFile::UvToml,
+                    ),
                     field.example,
-                    parents,
-                    ConfigurationFile::PyprojectToml,
-                ),
-                field.example,
-            ));
-            output.push_str(&format_tab(
-                "uv.toml",
-                &format_header(
-                    field.scope,
+                ));
+            } else {
+                output.push_str(&format_tab(
+                    "pyproject.toml",
+                    &format_header(
+                        field.scope,
+                        field.example,
+                        parents,
+                        ConfigurationFile::PyprojectToml,
+                    ),
                     field.example,
-                    parents,
-                    ConfigurationFile::UvToml,
-                ),
-                field.example,
-            ));
+                ));
+                output.push_str(&format_tab(
+                    "uv.toml",
+                    &format_header(
+                        field.scope,
+                        field.example,
+                        parents,
+                        ConfigurationFile::UvToml,
+                    ),
+                    field.example,
+                ));
+            }
         }
         _ => {}
     }