Go: file syncer service framework (#16579)

### Summary

./ragflow_main --syncer to start file syncer


config yaml file has following config
```
file_syncer:
  max_concurrent_syncs: 4 # concurrent file sync threads
  sync_interval: 3 # check interval

```

---------

Signed-off-by: Jin Hai <haijin.chn@gmail.com>

internal/server/config.go

@@ -53,6 +53,7 @@ type Config struct {
 	DefaultSuperUser DefaultSuperUser       `mapstructure:"default_super_user"`
 	Language         string                 `mapstructure:"language"`
 	TaskExecutor     TaskExecutorConfig     `mapstructure:"task_executor"`
+	FileSyncer       FileSyncerConfig       `mapstructure:"file_syncer"`
 }
 
 // AdminConfig admin server configuration
@@ -76,6 +77,11 @@ type TaskExecutorConfig struct {
 	MessageQueueType string `mapstructure:"message_queue_type"`
 }
 
+type FileSyncerConfig struct {
+	MaxConcurrentSyncs int `mapstructure:"max_concurrent_syncs"`
+	SyncInterval       int `mapstructure:"sync_interval"`
+}
+
 // UserDefaultLLMConfig user default LLM configuration
 type UserDefaultLLMConfig struct {
 	DefaultModels DefaultModelsConfig `mapstructure:"default_models"`