message: storage: add BlobInfo::is_stargz/set_stargz()
  Add BlobInfo::is_stargz/set_stargz(), so we could explicitly detect
  blobs for stargz image instead of implicitly check that compression
  algorithm is gzip.
diff: @@ -83,6 +83,8 @@ pub struct BlobInfo {
  readahead_size: u32,
  /// Whether to validate blob data.
  validate_data: bool,
  + /// The blob is for an stargz image.
  + stargz: bool,
  /// V6: Version number of the blob metadata.
  metadata_version: u32,
  @@ -115,6 +117,7 @@ impl BlobInfo {
  readahead_offset: 0,
  readahead_size: 0,
  validate_data: false,
  + stargz: false,
  metadata_version: 0,
  metadata_offset: 0,
  metadata_compressed_size: 0,
  @@ -215,6 +218,17 @@ impl BlobInfo {
  self.validate_data = validate;
  }
  + /// Check whether this blob is for an stargz image.
  + pub fn is_stargz(&self) -> bool {
  + self.stargz
  + }
  +
  + /// Set whether the blob is for an stargz image.
  + pub fn set_stargz(&mut self, stargz: bool) {
  + self.stargz = stargz;
  + }
  +
  + /// Set metadata information for compressed blob.
  pub fn set_blob_metadata_info(&mut self, version: u32, offset: u64, compressed_size: u64) {
  self.metadata_version = version;
  self.metadata_offset = offset;
  