storage/gcp: allow json key to be used for GCS access (#1088)

config.dev.toml

@@ -182,6 +182,16 @@ SingleFlightType = "memory"
         # Env override: ATHENS_STORAGE_GCP_BUCKET
         Bucket = "MY_GCP_BUCKET"
 
+        # JSONKey allows Athens to be run outside of GCP
+        # but still be able to access GCS. If you are 
+        # running Athens inside GCP, you will most 
+        # likely not need this as GCP figures out 
+        # internal authentication between products for you. 
+        # Pro tip: if you are pasting this as a JSON inside a string,
+        # make sure you escape "\n" by making it "\\n".
+        # Env override: ATHENS_STORAGE_GCP_JSON_KEY
+        JSONKey = "SERVICE_ACCOUNT_KEY"
+
     [Storage.Minio]
         # Endpoint for Minio storage
         # Env override: ATHENS_MINIO_ENDPOINT