Remove use of deprecated GoogleCredential class

[rtyley]

What does this change?

This change is only related to the Google-Group-checking code in play-googleauth, which was using a deprecated class:

The com.google.api.client.googleapis.auth.oauth2.GoogleCredential class in https://github.com/googleapis/google-api-java-client was deprecated in May 2019 by googleapis/google-api-java-client#1258 ...the deprecation advice recommends using the classes from the new library: https://github.com/googleapis/google-auth-library-java

This commit switches to using the new com.google.auth.oauth2.ServiceAccountCredentials class from the
google-auth-library-oauth2-http artifact. Given this new class, our custom com.gu.googleauth.GoogleServiceAccount Scala case class is no longer necessary and I've removed it, in favour of the user passing us an instance of ServiceAccountCredentials - which is actually easier for a user to generate! Here's an example of how you make and use
an instance of the new credentials:

import org.apache.commons.io.Charsets.UTF_8
import org.apache.commons.io.IOUtils
import com.google.auth.oauth2.ServiceAccountCredentials
import com.gu.googleauth.GoogleGroupChecker

val impersonatedUser: String = ... // email address of the 'impersonated' account
val serviceAccountCert: String = ... // from Google Developers Console
val credentials  =
  ServiceAccountCredentials.fromStream(IOUtils.toInputStream(serviceAccountCert, UTF_8))

val groupChecker = new GoogleGroupChecker(impersonatedUser, credentials)

In Ophan, our old code to produce the custom com.gu.googleauth.GoogleServiceAccount instance actually invoked the now deprecated com.google.api.client.googleapis.auth.oauth2.GoogleCredential class - we'd been stuck on an old version of play-googleauth for some time because we have deprecation warnings set as fatal in our sbt build, and updating to later versions of play-googleauth would have meant accepting the updated google-api-client dependency where the class was fatally deprecated.

How to test

Using the https://github.com/guardian/ophan/pull/3974, I've checked that I can still login using the new code in play-googleauth.