Add admin get/set config keys API (#6113)

This PR adds two new admin APIs in Minio server and madmin package:
- GetConfigKeys(keys []string) ([]byte, error)
- SetConfigKeys(params map[string]string) (err error)

A key is a path in Minio configuration file, (e.g. notify.webhook.1)

The user will always send a string value when setting it in the config file,
the API will know how to convert the value to the appropriate type. The user
is also able to set a raw json.

Before setting a new config, Minio will validate all fields and try to connect
to notification targets if available.

cmd/api-errors.go

@@ -290,6 +290,7 @@ const (
 	ErrEvaluatorBindingDoesNotExist
 	ErrInvalidColumnIndex
 	ErrMissingHeaders
+	ErrAdminConfigNotificationTargetsFailed
 )
 
 // error code to APIError structure, these fields carry respective
@@ -886,6 +887,11 @@ var errorCodeResponse = map[APIErrorCode]APIError{
 		Description:    "JSON configuration provided has objects with duplicate keys",
 		HTTPStatusCode: http.StatusBadRequest,
 	},
+	ErrAdminConfigNotificationTargetsFailed: {
+		Code:           "XMinioAdminNotificationTargetsTestFailed",
+		Description:    "Configuration update failed due an unsuccessful attempt to connect to one or more notification servers",
+		HTTPStatusCode: http.StatusBadRequest,
+	},
 	ErrAdminCredentialsMismatch: {
 		Code:           "XMinioAdminCredentialsMismatch",
 		Description:    "Credentials in config mismatch with server environment variables",
@@ -1443,7 +1449,7 @@ func toAPIErrorCode(err error) (apiErr APIErrorCode) {
 		apiErr = ErrKMSNotConfigured
 	case crypto.ErrKMSAuthLogin:
 		apiErr = ErrKMSAuthFailure
-	case context.Canceled, context.DeadlineExceeded:
+	case errOperationTimedOut, context.Canceled, context.DeadlineExceeded:
 		apiErr = ErrOperationTimedOut
 	}
 	switch err {