Add GitHub token environment variable support for Copilot (#31392)

Add support for environment variables as authentication alternatives to
OAuth flow for Copilot. Closes #31172

We can include the token in HTTPS request headers to hopefully resolve
the rate limiting issue in #9483. This change will be part of a separate
PR.

Release Notes:

- Added support for manually providing an OAuth token for GitHub Copilot
Chat by assigning the GH_COPILOT_TOKEN environment variable

---------

Co-authored-by: Bennet Bo Fenner <bennetbo@gmx.de>

crates/language_models/src/provider/copilot_chat.rs

@@ -863,6 +863,13 @@ impl Render for ConfigurationView {
                                         copilot::initiate_sign_in(window, cx)
                                     })),
                             )
+                            .child(
+                                Label::new(
+                                    format!("You can also assign the {} environment variable and restart Zed.", copilot::copilot_chat::COPILOT_OAUTH_ENV_VAR),
+                                )
+                                .size(LabelSize::Small)
+                                .color(Color::Muted),
+                            )
                     }
                 },
                 None => v_flex().gap_6().child(Label::new(ERROR_LABEL)),