Update Awaitable.Threading.cs

Expose `MainThreadSynchronizationContext` so we can use in `Task` and similar API that can accept `SynchronizationContext` to return to main thread Add helper extension method to `Task` for conveniently scheduling task in one line ```C# var fireUser = await FirebaseAuth.DefaultInstance.SignInWithCredentialAsync(GoogleAuthProvider.GetCredential(user.IdToken,null)); await Awaitable.MainThreadAsync(); Debug.Log(fireUser); // Instead Debug.Log(await FirebaseAuth.DefaultInstance.SignInWithCredentialAsync(GoogleAuthProvider.GetCredential(user.IdToken,null)).OnMainThread()); ```