Firebase Realtime Database - how to manage database rules using REST API

I would like to get/set realtime database rules using Rest API however no tutorial is working for me. I try to do it like that:

• I copied the url to my realtime database which is in Europe https://my-project-database.europe-west1.firebasedatabase.app/
• I copied the java code from this tutorial: https://firebase.google.com/docs/database/rest/auth which was supposed to give me the access token

    val googleCred = GoogleCredential.fromStream(File("/path/to/my/key.json").inputStream())
    val scoped = googleCred.createScoped(
        Arrays.asList( // or use firebase.database.readonly for read-only access
            "https://www.googleapis.com/auth/firebase.database",
            "https://www.googleapis.com/auth/userinfo.email"
        )
    )
    scoped.refreshToken()
    val token = scoped.accessToken
    println(token)

However the token looks very strange with a long string of dots at the end like (it's related to: Why am I getting a JWT with a bunch of periods/dots back from Google OAuth?)

ya29.c.Kp8BCgi0lxWtUt-_[Normal JWT stuff, redacted for security]yVvGk...............................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................

• I did HTTP GET to address like https://my-project-database.europe-west1.firebasedatabase.app/.settings/rules.json?access_token=$token and I got 401 UNAUTHORIZED

I assume it's due to the fact that I use the whole of this strange token full of dots as the access_token variable. So now I have questions how to transform it and use as the access_token to make it work

EDIT: I created this gist although it's in python it has exactly the same problem. How to make it work ? https://gist.github.com/solveretur/86d53a9c0221f096c38c3ef8f70a8dbd