Adds X-Storefront request header for App Store Storefront (#3405)

### Motivation

Allow backend to determine Storefront of customer

### Description

- Adds `X-Storefront` as a default header
- Uses `StoreKit 1` implementation since its not `async`
- `StoreKit 2` implementation is `async` and would require a some
restructuring that we don't need right now

---------

Co-authored-by: NachoSoto <ignaciosoto90@gmail.com>

Sources/Networking/HTTPClient/HTTPClient.swift

@@ -225,6 +225,10 @@ private extension HTTPClient {
             "X-Is-Sandbox": "\(self.systemInfo.isSandbox)"
         ]
 
+        if let storefront = self.systemInfo.storefront {
+            headers["X-Storefront"] = storefront.countryCode
+        }
+
         if let platformFlavorVersion = self.systemInfo.platformFlavorVersion {
             headers["X-Platform-Flavor-Version"] = platformFlavorVersion
         }