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.

OCI image layers have traditionally been packaged as tar archives which
must be unpacked after downloading.

Read-only compressed filesystems such as squashfs and erofs present viable
alternatives such that the unpack step can be skipped since they are
directly mountable and overlayfs compatible.

squashfs is older but has wider platform support (Linux, *BSD and Windows).
erofs is newer and has strong community support but very Linux-specific.

Build tools will now produce these new layers directly instead of tar
archives.

Additional reading:
https://www.cyphar.com/blog/post/20190121-ociv2-images-i-tar
https://groups.google.com/a/opencontainers.org/g/dev/c/Zk3yf45HIdA?pli=1

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

Author: rchincha

layer.md

@@ -20,6 +20,8 @@ This section defines the `application/vnd.oci.image.layer.v1.tar`, `application/
 
 - Layer Changesets for the [media type](media-types.md) `application/vnd.oci.image.layer.v1.tar` MUST be packaged in [tar archive][tar-archive].
 - Layer Changesets for the [media type](media-types.md) `application/vnd.oci.image.layer.v1.tar` MUST NOT include duplicate entries for file paths in the resulting [tar archive][tar-archive].
+- Layer Changesets for the [media type](media-types.md) `application/vnd.oci.image.layer.v1.squashfs` MUST be packaged in [squashfs file][squashfs].
+- Layer Changesets for the [media type](media-types.md) `application/vnd.oci.image.layer.v1.tar` MUST be packaged in [erofs file][erofs].
 
 ## Change Types
 
@@ -342,3 +344,5 @@ Implementations SHOULD NOT upload layers tagged with this media type; however, s
 [rfc1952_2]: https://tools.ietf.org/html/rfc1952
 [tar-archive]: https://en.wikipedia.org/wiki/Tar_(computing)
 [rfc8478]: https://tools.ietf.org/html/rfc8478
+[squashfs]: https://github.com/plougher/squashfs-tools
+[erofs]: https://erofs.docs.kernel.org/

specs-go/v1/mediatype.go

@@ -45,6 +45,14 @@ const (
 	// MediaTypeImageLayerZstd is the media type used for zstd compressed
 	// layers referenced by the manifest.
 	MediaTypeImageLayerZstd = "application/vnd.oci.image.layer.v1.tar+zstd"
+
+	// MediaTypeSquashfs specifies the media type as squashfs filesystem layer
+	// https://github.com/plougher/squashfs-tools
+	MediaTypeSquashfs = "application/vnd.oci.image.layer.v1.squashfs"
+
+	// MediaTypeErofs specifies the media type as erofs filesystem layer
+	// https://www.iana.org/assignments/media-types/application/vnd.erofs
+	MediaTypeErofs = "application/vnd.oci.image.layer.v1.erofs"
 )
 
 // Non-distributable layer media-types.