Adds ability to track Firebase Auth UI per request (firebase#251)

* Adds ability to track Firebase Auth UI per request

Adds internal method to FIRAuth which will be accessed in Auth UI via an extension.

Example request header header with Firebase Auth Core Marker:
X-Client-Version: iOS/FirebaseSDK/0.1.1/FirebaseCore-iOS

Author: protocol86

Firebase/Auth/Source/FIRAuth.m

@@ -202,6 +202,11 @@ @interface FIRAuth ()
  */
 @property(nonatomic, copy, readonly) NSString *firebaseAppId;
 
+/** @property additionalFrameworkMarker
+    @brief Additional framework marker that will be added as part of the header of every request.
+ */
+@property(nonatomic, copy, nullable) NSString *additionalFrameworkMarker;
+
 /** @fn initWithApp:
     @brief Creates a @c FIRAuth instance associated with the provided @c FIRApp instance.
     @param app The application to associate the auth instance with.
@@ -945,15 +950,29 @@ - (void)removeIDTokenDidChangeListener:(FIRIDTokenDidChangeListenerHandle)listen
 }
 
 - (void)useAppLanguage {
-  _requestConfiguration.languageCode = [NSBundle mainBundle].preferredLocalizations.firstObject;
+  dispatch_sync(FIRAuthGlobalWorkQueue(), ^{
+    _requestConfiguration.languageCode = [NSBundle mainBundle].preferredLocalizations.firstObject;
+  });
 }
 
 - (nullable NSString *)languageCode {
   return _requestConfiguration.languageCode;
 }
 
 - (void)setLanguageCode:(nullable NSString *)languageCode {
-  _requestConfiguration.languageCode = [languageCode copy];
+  dispatch_sync(FIRAuthGlobalWorkQueue(), ^{
+    _requestConfiguration.languageCode = [languageCode copy];
+  });
+}
+
+- (NSString *)additionalFrameworkMarker {
+  return _requestConfiguration.additionalFrameworkMarker;
+}
+
+- (void)setAdditionalFrameworkMarker:(NSString *)additionalFrameworkMarker {
+  dispatch_sync(FIRAuthGlobalWorkQueue(), ^{
+    _requestConfiguration.additionalFrameworkMarker = [additionalFrameworkMarker copy];
+  });
 }
 
 #if TARGET_OS_IOS

Firebase/Auth/Source/RPCs/FIRAuthBackend.m

@@ -76,6 +76,11 @@
  */
 static NSString *const kFirebaseLocalHeader = @"X-Firebase-Locale";
 
+/** @var kFirebaseAuthCoreFrameworkMarker
+    @brief The marker in the HTTP header that indicates the request comes from Firebase Auth Core.
+ */
+static NSString *const kFirebaseAuthCoreFrameworkMarker = @"FirebaseCore-iOS";
+
 /** @var kJSONContentType
     @brief The value of the HTTP content-type header for JSON payloads.
  */
@@ -499,8 +504,11 @@ - (void)asyncPostToURLWithRequestConfiguration:(FIRAuthRequestConfiguration *)re
                                NSError *_Nullable))handler {
   NSMutableURLRequest *request = [NSMutableURLRequest requestWithURL:URL];
   [request setValue:contentType forHTTPHeaderField:@"Content-Type"];
-  NSString *clientVersion =
-      [NSString stringWithFormat:@"iOS/FirebaseSDK/%s", FirebaseAuthVersionString];
+  NSString *additionalFrameworkMarker = requestConfiguration.additionalFrameworkMarker ?:
+      kFirebaseAuthCoreFrameworkMarker;
+  NSString *clientVersion = [NSString stringWithFormat:@"iOS/FirebaseSDK/%s/%@",
+                                                       FirebaseAuthVersionString,
+                                                       additionalFrameworkMarker];
   [request setValue:clientVersion forHTTPHeaderField:kClientVersionHeader];
   NSString *bundleID = [[NSBundle mainBundle] bundleIdentifier];
   [request setValue:bundleID forHTTPHeaderField:kIosBundleIdentifierHeader];

Firebase/Auth/Source/RPCs/FIRAuthRequestConfiguration.h

@@ -35,6 +35,11 @@ NS_ASSUME_NONNULL_BEGIN
  */
 @property(nonatomic, copy, nullable) NSString *languageCode;
 
+/** @property additionalFrameworkMarker
+    @brief Additional framework marker that will be added as part of the header of every request.
+ */
+@property(nonatomic, copy, nullable) NSString *additionalFrameworkMarker;
+
 - (nullable instancetype)init NS_UNAVAILABLE;
 
 /** @fn initWithRequestClass:APIKey:authLanguage: