Add userinfo.email scope

[mtmendonca]

Allows GKE RBAC authentication using the provided service account's email instead of its ID.
Based on GKE docs the token used for GKE authentication needs the userinfo.email scope for a ClusterRoleBinding with User: service-accountl@project-.iam.gserviceaccount.com to work, otherwise, the service account's unique ID is required.

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[mtmendonca]

@googlebot I signed it!

[bharathkkb]

Thanks for the PR