There is no sign-out button provided by google.
If you would like extra buttons in the same style as google’s, you will need to create them yourself.
But it’s a lot easier than it sounds, there are many resources provided by Google to show you how to create a button that looks just like a Google button. Take a look over here:
Here is a little example I made in microsoft paint to show you the kind of stuff you can make!
You can make your button look the same as
<com.google.android.gms.common.SignInButton>, adding a predefined style to your button with the xml tag
Note that its a FirebaseUI style and you can put the Button text that you want.