feat: [firebase_ui_auth] [firebase_ui_oauth] Allow for custom labels to be used for custom OAuth providers (#9858)

* Added `label` field to `ThemedOAuthProviderButtonStyle`

* Added comment

* Code simplification

* Formatting

* Removed dependency overrides

Author: Rexios80

packages/firebase_ui_auth/lib/src/widgets/internal/oauth_provider_button.dart

@@ -110,7 +110,8 @@ class OAuthProviderButton extends StatelessWidget {
           ),
           label: variant == OAuthButtonVariant.icon
               ? ''
-              : resolveProviderButtonLabel(provider.providerId, labels),
+              : provider.style.label ??
+                  resolveProviderButtonLabel(provider.providerId, labels),
           auth: auth,
         );
 

packages/firebase_ui_oauth/lib/src/oauth_provider_button_style.dart

@@ -46,6 +46,11 @@ abstract class ThemedOAuthProviderButtonStyle {
   double get iconPadding => 0;
   String get assetsPackage;
 
+  /// A custom label string.
+  ///
+  /// Required for custom OAuth providers.
+  String? get label => null;
+
   /// {@macro ui.oauth.themed_oauth_provider_button_style}
   const ThemedOAuthProviderButtonStyle();