copilot: Support HTTP/HTTPS proxy for Copilot language server (#24364)

Closes #6701 (one of the top ranking issues as of writing)

Adds the ability to specify an HTTP/HTTPS proxy to route Copilot code
completion API requests through. This should fix copilot functionality
in restricted network environments (where such a proxy is required) but
also opens up the ability to point copilot code completion requests at
your own local LLM, using e.g.:
- https://github.com/jjleng/copilot-proxy
- https://github.com/bernardo-bruning/ollama-copilot/tree/master

External MITM-proxy tools permitting, this can serve as a stop-gap to
allow local LLM code completion in Zed until a proper OpenAI-compatible
local code completions provider is implemented. With this in mind, in
this PR I've added separate `settings.json` variables to configure a
proxy server _specific to the code completions provider_ instead of
using the global `proxy` setting, to allow for cases like this where we
_only_ want to proxy e.g. the Copilot requests, but not all outgoing
traffic from the application.

Currently, two new settings are added:
- `inline_completions.copilot.proxy`: Proxy server URL (HTTP and HTTPS
schemes supported)
- `inline_completions.copilot.proxy_no_verify`: Whether to disable
certificate verification through the proxy

Example:
```js
"features": {
  "inline_completion_provider": "copilot"
},
"show_completions_on_input": true,
// New:
"inline_completions": {
  "copilot": {
    "proxy": "http://example.com:15432",
    "proxy_no_verify": true
  }
}
```


Release Notes:

- Added the ability to specify an HTTP/HTTPS proxy for Copilot.

---------

Co-authored-by: Marshall Bowers <git@maxdeviant.com>

crates/language/src/language_settings.rs

@@ -234,6 +234,8 @@ pub struct EditPredictionSettings {
     pub disabled_globs: Vec<GlobMatcher>,
     /// Configures how edit predictions are displayed in the buffer.
     pub mode: EditPredictionsMode,
+    /// Settings specific to GitHub Copilot.
+    pub copilot: CopilotSettings,
 }
 
 /// The mode in which edit predictions should be displayed.
@@ -248,6 +250,14 @@ pub enum EditPredictionsMode {
     EagerPreview,
 }
 
+#[derive(Clone, Debug, Default)]
+pub struct CopilotSettings {
+    /// HTTP/HTTPS proxy to use for Copilot.
+    pub proxy: Option<String>,
+    /// Disable certificate verification for proxy (not recommended).
+    pub proxy_no_verify: Option<bool>,
+}
+
 /// The settings for all languages.
 #[derive(Debug, Clone, Default, PartialEq, Serialize, Deserialize, JsonSchema)]
 pub struct AllLanguageSettingsContent {
@@ -465,6 +475,23 @@ pub struct EditPredictionSettingsContent {
     /// Provider support required.
     #[serde(default)]
     pub mode: EditPredictionsMode,
+    /// Settings specific to GitHub Copilot.
+    #[serde(default)]
+    pub copilot: CopilotSettingsContent,
+}
+
+#[derive(Clone, Debug, Default, Serialize, Deserialize, JsonSchema, PartialEq)]
+pub struct CopilotSettingsContent {
+    /// HTTP/HTTPS proxy to use for Copilot.
+    ///
+    /// Default: none
+    #[serde(default)]
+    pub proxy: Option<String>,
+    /// Disable certificate verification for the proxy (not recommended).
+    ///
+    /// Default: false
+    #[serde(default)]
+    pub proxy_no_verify: Option<bool>,
 }
 
 /// The settings for enabling/disabling features.
@@ -1064,6 +1091,16 @@ impl settings::Settings for AllLanguageSettings {
             .map(|globs| globs.iter().collect())
             .ok_or_else(Self::missing_default)?;
 
+        let mut copilot_settings = default_value
+            .edit_predictions
+            .as_ref()
+            .map(|settings| settings.copilot.clone())
+            .map(|copilot| CopilotSettings {
+                proxy: copilot.proxy,
+                proxy_no_verify: copilot.proxy_no_verify,
+            })
+            .unwrap_or_default();
+
         let mut file_types: HashMap<Arc<str>, GlobSet> = HashMap::default();
 
         for (language, suffixes) in &default_value.file_types {
@@ -1096,6 +1133,22 @@ impl settings::Settings for AllLanguageSettings {
                 }
             }
 
+            if let Some(proxy) = user_settings
+                .edit_predictions
+                .as_ref()
+                .and_then(|settings| settings.copilot.proxy.clone())
+            {
+                copilot_settings.proxy = Some(proxy);
+            }
+
+            if let Some(proxy_no_verify) = user_settings
+                .edit_predictions
+                .as_ref()
+                .and_then(|settings| settings.copilot.proxy_no_verify)
+            {
+                copilot_settings.proxy_no_verify = Some(proxy_no_verify);
+            }
+
             // A user's global settings override the default global settings and
             // all default language-specific settings.
             merge_settings(&mut defaults, &user_settings.defaults);
@@ -1147,6 +1200,7 @@ impl settings::Settings for AllLanguageSettings {
                     .filter_map(|g| Some(globset::Glob::new(g).ok()?.compile_matcher()))
                     .collect(),
                 mode: edit_predictions_mode,
+                copilot: copilot_settings,
             },
             defaults,
             languages,