message: rafs: public some files of RafsConfig
  So nydusctl can directly access those fields and print them.
diff: @@ -63,12 +63,12 @@ fn default_prefetch_all() -> bool {
  #[derive(Clone, Default, Deserialize)]
  pub struct FsPrefetchControl {
  #[serde(default)]
  - enable: bool,
  + pub enable: bool,
  #[serde(default = "default_threads_count")]
  - threads_count: usize,
  + pub threads_count: usize,
  #[serde(default = "default_merging_size")]
  // In unit of Bytes
  - merging_size: usize,
  + pub merging_size: usize,
  #[serde(default)]
  // In unit of Bytes. It sets a limit to prefetch bandwidth usage in order to
  // reduce congestion with normal user IO.
  @@ -78,7 +78,7 @@ pub struct FsPrefetchControl {
  // it will be raised to the chunk size.
  bandwidth_rate: u32,
  #[serde(default = "default_prefetch_all")]
  - prefetch_all: bool,
  + pub prefetch_all: bool,
  }
  /// Not everything can be safely exported from configuration.
  