feat(gcs): add application default credentials fallback support (#8161)

* feat(gcs): add application default credentials fallback support

* refactor

* Update weed/remote_storage/gcs/gcs_storage_client.go

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

---------

Co-authored-by: Chris Lu <chris.lu@gmail.com>
Co-authored-by: Chris Lu <chrislusf@users.noreply.github.com>
Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

weed/remote_storage/gcs/gcs_storage_client.go

@@ -38,26 +38,26 @@ func (s gcsRemoteStorageMaker) Make(conf *remote_pb.RemoteConf) (remote_storage.
 	googleApplicationCredentials := conf.GcsGoogleApplicationCredentials
 
 	if googleApplicationCredentials == "" {
-		found := false
-		googleApplicationCredentials, found = os.LookupEnv("GOOGLE_APPLICATION_CREDENTIALS")
-		if !found {
-			return nil, fmt.Errorf("need to specific GOOGLE_APPLICATION_CREDENTIALS env variable")
+		if creds, found := os.LookupEnv("GOOGLE_APPLICATION_CREDENTIALS"); found {
+			googleApplicationCredentials = creds
+		} else {
+			glog.Warningf("no GOOGLE_APPLICATION_CREDENTIALS env variable found, falling back to Application Default Credentials")
 		}
 	}
 
 	projectID := conf.GcsProjectId
 	if projectID == "" {
-		found := false
-		projectID, found = os.LookupEnv("GOOGLE_CLOUD_PROJECT")
-		if !found {
-			glog.Warningf("need to specific GOOGLE_CLOUD_PROJECT env variable")
+		if pid, found := os.LookupEnv("GOOGLE_CLOUD_PROJECT"); found {
+			projectID = pid
+		} else {
+			glog.Warningf("need to specify GOOGLE_CLOUD_PROJECT env variable")
 		}
 	}
 
-	googleApplicationCredentials = util.ResolvePath(googleApplicationCredentials)
-
 	var clientOpts []option.ClientOption
+
 	if googleApplicationCredentials != "" {
+		googleApplicationCredentials = util.ResolvePath(googleApplicationCredentials)
 		var data []byte
 		var err error
 		if strings.HasPrefix(googleApplicationCredentials, "{") {