Update Awaitable.Threading.cs

[Thaina]

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

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());

[qingshu]

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