[FR] please add onSignIn to auth function trigger

### [REQUIRED] Version info
9.15.0

**node:**
16.18.1

**firebase-functions:**
0.9.0

**firebase-tools:**
11.18.0

**firebase-admin:**
11.2.0

### [REQUIRED] Expected behavior

Have function to trigger after user signin like `onCreate` function

```js
functions.auth.user().onSignIn((user,context) => {
});
```

Because beforeSignIn is blocking function and trigger just before user successfully signed in. It not a proper function to take a log and monitoring user signing in or taking any complicate action. I want this function to just notify the server and can run the long running task