[Auth] Await header value from underlying queue (#13647)

Author: ncooke3

FirebaseAuth/Sources/Swift/Backend/AuthBackend.swift

@@ -109,7 +109,11 @@ class AuthBackend: NSObject {
     request.setValue(Bundle.main.bundleIdentifier, forHTTPHeaderField: "X-Ios-Bundle-Identifier")
     request.setValue(requestConfiguration.appID, forHTTPHeaderField: "X-Firebase-GMPID")
     if let heartbeatLogger = requestConfiguration.heartbeatLogger {
-      request.setValue(heartbeatLogger.headerValue(), forHTTPHeaderField: "X-Firebase-Client")
+      // The below call synchronously dispatches to a queue. To avoid blocking
+      // the shared concurrency queue, `async let` will spawn the process on
+      // a separate thread.
+      async let heartbeatsHeaderValue = heartbeatLogger.headerValue()
+      await request.setValue(heartbeatsHeaderValue, forHTTPHeaderField: "X-Firebase-Client")
     }
     request.httpMethod = requestConfiguration.httpMethod
     let preferredLocalizations = Bundle.main.preferredLocalizations