Add auth to jib-gradle-plugin.

[coollog]

Add configuration to use raw credentials directly in the plugin configuration as such:

jib {
  from {
    image = 'gcr.io/my-gcp-project/my-base-image'
    auth {
      username = SOME_USERNAME
      password = SOME_PASSWORD
    }
  }
}

The auth should be specific to the base image. SOME_USERNAME and SOME_PASSWORD can be a raw string, but should be defined in gradle.properties and not committed to VC.