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/identity/openid/jwt_test.go

@@ -53,7 +53,7 @@ func TestJWT(t *testing.T) {
 		}
 	}
 
-	u1, err := xnet.ParseURL("http://localhost:8443")
+	u1, err := xnet.ParseHTTPURL("http://localhost:8443")
 	if err != nil {
 		t.Fatal(err)
 	}