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/config/policy/opa/config.go

@@ -124,7 +124,7 @@ func LookupConfig(kv config.KVS, transport *http.Transport, closeRespFn func(io.
 		authToken = env.Get(EnvPolicyOpaAuthToken, kv.Get(AuthToken))
 	}
 
-	u, err := xnet.ParseURL(opaURL)
+	u, err := xnet.ParseHTTPURL(opaURL)
 	if err != nil {
 		return args, err
 	}