Add cimfs differ and snapshotter

Details about CimFs project are discussed in #8346

Signed-off-by: Amit Barve <ambarve@microsoft.com>

vendor/github.com/Microsoft/hcsshim/pkg/cimfs/format/doc.go

@@ -0,0 +1,4 @@
+// format package maintains some basic structures to allows us to read header of a cim file. This is mostly
+// required to understand the region & objectid files associated with a particular cim. Otherwise, we don't
+// need to parse the cim format.
+package format

vendor/github.com/Microsoft/hcsshim/pkg/cimfs/format/format.go

@@ -0,0 +1,61 @@
+//go:build windows
+// +build windows
+
+package format
+
+import "github.com/Microsoft/go-winio/pkg/guid"
+
+const (
+	RegionFileName   = "region"
+	ObjectIDFileName = "objectid"
+)
+
+// Magic specifies the magic number at the beginning of a file.
+type Magic [8]uint8
+
+var MagicValue = Magic([8]uint8{'c', 'i', 'm', 'f', 'i', 'l', 'e', '0'})
+
+type Version struct {
+	Major, Minor uint32
+}
+
+var CurrentVersion = Version{3, 0}
+
+var MinSupportedVersion = Version{2, 0}
+
+type FileType uint8
+
+// RegionOffset encodes an offset to objects as index of the region file
+// containing the object and the byte offset within that file.
+type RegionOffset uint64
+
+// CommonHeader is the common header for all CIM-related files.
+type CommonHeader struct {
+	Magic        Magic
+	HeaderLength uint32
+	Type         FileType
+	Reserved     uint8
+	Reserved2    uint16
+	Version      Version
+	Reserved3    uint64
+}
+
+type RegionSet struct {
+	ID        guid.GUID
+	Count     uint16
+	Reserved  uint16
+	Reserved1 uint32
+}
+
+// FilesystemHeader is the header for a filesystem file.
+//
+// The filesystem file points to the filesystem object inside a region
+// file and specifies regions sets.
+type FilesystemHeader struct {
+	Common           CommonHeader
+	Regions          RegionSet
+	FilesystemOffset RegionOffset
+	Reserved         uint32
+	Reserved1        uint16
+	ParentCount      uint16
+}