message: nydus-snapshotter: add PrefetchAll config in DaemonConfig
  We introduced 'prefetch_all' option to control if nydus should prefetch
  all blobs in background. So add this option to snapshotter, that we
  could enable/disable it.
diff: @@ -32,6 +32,7 @@ type DaemonConfig struct {
  EnableXattr bool `json:"enable_xattr,omitempty"`
  FSPrefetch struct {
  Enable bool `json:"enable"`
  + PrefetchAll bool `json:"prefetch_all"`
  ThreadsCount int `json:"threads_count"`
  MergingSize int `json:"merging_size"`
  } `json:"fs_prefetch,omitempty"`
  