Extend further validation of config values (#8469)

- This PR allows config KVS to be validated properly
  without being affected by ENV overrides, rejects
  invalid values during set operation

- Expands unit tests and refactors the error handling
  for notification targets, returns error instead of
  ignoring targets for invalid KVS

- Does all the prep-work for implementing safe-mode
  style operation for MinIO server, introduces a new
  global variable to toggle safe mode based operations
  NOTE: this PR itself doesn't provide safe mode operations

cmd/gateway-startup-msg.go

@@ -28,9 +28,8 @@ import (
 func printGatewayStartupMessage(apiEndPoints []string, backendType string) {
 	strippedAPIEndpoints := stripStandardPorts(apiEndPoints)
 	// If cache layer is enabled, print cache capacity.
-	cacheObjectAPI := newCacheObjectsFn()
-	if cacheObjectAPI != nil {
-		printCacheStorageInfo(cacheObjectAPI.StorageInfo(context.Background()))
+	if globalCacheObjectAPI != nil {
+		printCacheStorageInfo(globalCacheObjectAPI.StorageInfo(context.Background()))
 	}
 	// Prints credential.
 	printGatewayCommonMsg(strippedAPIEndpoints)