GCR authentication - Add support to authenticate via a json key file

[snps-elie]

I have all my images pushed to GCR. I tested OpsSight with access tokens and it works as expected. The problem is that access tokens are not the recommended way to authenticate with GCR. "Access tokens are short-lived tokens that provide read/write access to your Google Cloud Platform resources."

Google usually recommend using json key files for service accounts to authenticate. https://cloud.google.com/container-registry/docs/advanced-authentication

Can you add support for this?