message: Snapshotter: add verifier to fs
  Fixes nil deref panics:
  ```
  pkg/filesystem/nydus/fs.go:128 +0x1f5
  pkg/filesystem/nydus.(*filesystem).Mount(0xc0002a0000, 0x3f151a0, 0xc00025ae10, 0xc0013bbd58, 0x2, 0xc000cf2120, 0x0, 0x0)
  pkg/signature/signature.go:57 +0x1a1
  pkg/signature.(*Verifier).Verify(0x0, 0xc000cf2120, 0xc00060f740, 0x51, 0x0, 0x0)
  ```
diff: @@ -8,6 +8,7 @@ import (
  "contrib/nydus-snapshotter/config"
  "contrib/nydus-snapshotter/pkg/filesystem/nydus"
  "contrib/nydus-snapshotter/pkg/filesystem/stargz"
  + "contrib/nydus-snapshotter/pkg/signature"
  "contrib/nydus-snapshotter/snapshot"
  )
  @@ -31,12 +32,18 @@ func init() {
  return nil, errors.New("failed to fillup nydus configuration with defaults")
  }
  + verifier, err := signature.NewVerifier(cfg.PublicKeyFile, cfg.ValidateSignature)
  + if err != nil {
  + return nil, errors.Wrap(err, "failed to initialize verifier")
  + }
  +
  fs, err := nydus.NewFileSystem(
  ic.Context,
  nydus.WithNydusdBinaryPath(cfg.NydusdBinaryPath),
  nydus.WithMeta(cfg.RootDir),
  nydus.WithDaemonConfig(cfg.DaemonCfg),
  nydus.WithSharedDaemon(cfg.SharedDaemon),
  + nydus.WithVerifier(verifier),
  )
  if err != nil {
  return nil, errors.Wrap(err, "failed to initialize nydus filesystem")
  