Provide a method to get authenticated from GCP

There isn't an authentication mechanism in the provided Loki configuration. The Loki GCS authentication is handled via the GCS SDK which is typically done with environment variables(For example:GOOGLE_APPLICATION_CREDENTIALS).
Refer to https://stackoverflow.com/questions/65232062/can-i-setup-a-single-store-on-gcs-and-have-multiple-local-instances-of-loki-read.

The loki-operator should provide a method to handle GCS authentication. we can refer to docker registry which supports various Objective Storage too. It uses REGISTRY_STORAGE_GCS_KEYFILE to get authenticated from GCP. Refer to https://docs.openshift.com/container-platform/4.10/registry/configuring_registry_storage/configuring-registry-storage-gcp-user-infrastructure.html