Implement Linux support via UNIX socket proxy (#93)

This is a companion PR to mozilla-mobile/mozilla-vpn-client#9952 that
implements Linux support by connecting to the SOCKS proxy as a systemd
service. Basically all we do here is check if the platform is `linux`
and set the bypass proxy to `{type: "socks", host:
"file:/var/run/mozillavpn.proxy", port: 1234}` The port number here
isn't used, but it seems to be required in order to be parsed as a valid
proxy.

Update! Now it supports connecting to the Windows proxy service too.
Similar logic, we just check the platform and generate a fixed proxy
config.

src/background/proxyHandler/proxyHandler.js

@@ -34,6 +34,31 @@ export class ProxyHandler extends Component {
   constructor(receiver, controller) {
     super(receiver);
     this.controller = controller;
+    browser.runtime.getPlatformInfo((info) => {
+      this.#mPlatformOs = info.os;
+      if (info.os === "linux") {
+        // Linux provides bypass via a UNIX domain socket.
+        this.#mLocalProxyInfo.value = [
+          {
+            type: "socks",
+            host: "file:/var/run/mozillavpn.proxy",
+            port: 1234,
+          },
+        ];
+      } else if (info.os === "win") {
+        // Windows uses a service on a well-known TCP port.
+        this.#mLocalProxyInfo.value = [
+          {
+            type: "socks",
+            host: "localhost",
+            port: 8123,
+          },
+        ];
+      } else {
+        // Not supported on this platform
+        this.#mLocalProxyInfo.value = [];
+      }
+    });
   }
 
   /** @type {VPNState | undefined} */
@@ -46,6 +71,7 @@ export class ProxyHandler extends Component {
   #mProxyMap = property(new Map());
   #mLocalProxyInfo = property([]);
   #mCurrentExitRelays = property([]);
+  #mPlatformOs = "unknown";
 
   /** @type {IBindable<Map<String, SiteContext>>} */
   get siteContexts() {
@@ -108,9 +134,6 @@ export class ProxyHandler extends Component {
    */
   processClientStateChanges(vpnState) {
     console.log(`Processing client state change ${vpnState}`);
-    this.#mLocalProxyInfo.value = vpnState.loophole
-      ? [ProxyUtils.parseProxy(vpnState.loophole)]
-      : [];
 
     if (this.servers.length == 0) {
       console.log("No servers, unable to get exit location proxy info");