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/web-handlers.go

@@ -534,7 +534,7 @@ func (web *webAPIHandlers) SetAuth(r *http.Request, args *SetAuthArgs, reply *Se
 	prevCred := globalServerConfig.SetCredential(creds)
 
 	// Persist updated credentials.
-	if err = saveServerConfig(newObjectLayerFn(), globalServerConfig); err != nil {
+	if err = saveServerConfig(context.Background(), newObjectLayerFn(), globalServerConfig); err != nil {
 		// Save the current creds when failed to update.
 		globalServerConfig.SetCredential(prevCred)
 		logger.LogIf(context.Background(), err)