Add support for erofs and squashfs layer media types

OCI artifacts support has landed in various OCI specs v1.1.0 which
allows for arbitrary artifact types, small and large.

Large artifacts (even existing container images) pose a particular challenge that:
1. it takes too long to download
2. it takes too long to unpack

This PR **begins** to address 2. above.

squashfs (an older read-only) and erofs (newer) filesystems are already
compressed full filesystems, and overlayfs compatible.

Signed-off-by: Ramkumar Chinchani <rchincha@cisco.com>

Author: Ramkumar Chinchani

specs-go/v1/mediatype.go

@@ -45,6 +45,13 @@ const (
 	// MediaTypeImageLayerZstd is the media type used for zstd compressed
 	// layers referenced by the manifest.
 	MediaTypeImageLayerZstd = "application/vnd.oci.image.layer.v1.tar+zstd"
+
+	// MediaTypeErofs specifies the media type as erofs filesystem layer
+	// https://www.iana.org/assignments/media-types/application/vnd.erofs
+	MediaTypeErofs = "application/vnd.erofs"
+
+	// MediaTypeSquashfs specifies the media type as squashfs filesystem layer
+	MediaTypeSquashfs = "application/vnd.squashfs"
 )
 
 // Non-distributable layer media-types.