Add admin file inspector (#12635)

Download files from *any* bucket/path as an encrypted zip file.

The key is included in the response but can be separated so zip 
and the key doesn't have to be sent on the same channel.

Requires https://github.com/minio/pkg/pull/6

cmd/xl-storage.go

@@ -2284,3 +2284,32 @@ func (s *xlStorage) VerifyFile(ctx context.Context, volume, path string, fi File
 
 	return nil
 }
+
+func (s *xlStorage) StatInfoFile(ctx context.Context, volume, path string) (stat StatInfo, err error) {
+	volumeDir, err := s.getVolDir(volume)
+	if err != nil {
+		return stat, err
+	}
+
+	// Stat a volume entry.
+	if err = Access(volumeDir); err != nil {
+		if osIsNotExist(err) {
+			return stat, errVolumeNotFound
+		} else if isSysErrIO(err) {
+			return stat, errFaultyDisk
+		} else if osIsPermission(err) {
+			return stat, errVolumeAccessDenied
+		}
+		return stat, err
+	}
+	filePath := pathJoin(volumeDir, path)
+	if err := checkPathLength(filePath); err != nil {
+		return stat, err
+	}
+	st, _ := Lstat(filePath)
+	if st == nil {
+		return stat, errPathNotFound
+	}
+
+	return StatInfo{ModTime: st.ModTime(), Size: st.Size()}, nil
+}