Add X-Preferred-Locales header (#4008)

Added a new `X-Supported-Locales` header to be used in the new customer
center feature

Sources/Networking/HTTPClient/HTTPClient.swift

@@ -102,6 +102,9 @@ class HTTPClient {
 
     // Visible for tests
     var defaultHeaders: RequestHeaders {
+        let preferredLanguages = self.systemInfo.preferredLanguages.prefix(3).map {
+            $0.replacingOccurrences(of: "-", with: "_")
+        }.joined(separator: ",")
         var headers: RequestHeaders = [
             "content-type": "application/json",
             "X-Version": SystemInfo.frameworkVersion,
@@ -111,6 +114,7 @@ class HTTPClient {
             "X-Client-Version": SystemInfo.appVersion,
             "X-Client-Build-Version": SystemInfo.buildVersion,
             "X-Client-Bundle-ID": SystemInfo.bundleIdentifier,
+            "X-Preferred-Locales": preferredLanguages,
             "X-StoreKit2-Enabled": "\(self.systemInfo.storeKitVersion.isStoreKit2EnabledAndAvailable)",
             "X-StoreKit-Version": "\(self.systemInfo.storeKitVersion.effectiveVersion)",
             "X-Observer-Mode-Enabled": "\(self.systemInfo.observerMode)",