Set buttonStyle to be MainActor (#543)

GoogleSignInSwift/Sources/GoogleSignInButtonViewModel.swift

@@ -28,6 +28,7 @@ public class GoogleSignInButtonViewModel: ObservableObject {
 
   /// A computed property providing the button's size, colors, corner radius,
   /// and shadow based on this current view model's `SignInButtonStyle`.
+  @MainActor
   var buttonStyle: SwiftUIButtonStyle {
     return SwiftUIButtonStyle(style: style, state: state, scheme: scheme)
   }

GoogleSignInSwift/Tests/Unit/GoogleSignInButtonStylingTests.swift

@@ -17,6 +17,7 @@
 import XCTest
 @testable import GoogleSignInSwift
 
+@MainActor
 @available(iOS 13.0, macOS 10.15, *)
 class GoogleSignInButtonStylingTests: XCTestCase {
   private typealias ButtonViewModelInfo = (