ctr pull should unpack for default platform when transfer service is used

[djdongjin]

Currently if neither --all-platforms nor --platform is specified in ctr i pull, it will pull using the default platform, but skip the unpack step (if using transfer service). However, we should apply the default platform to unpack as well.

# in main branch, no any snapshot is generated
containerd [main]  ➜ sudo ctr i pull --snapshotter=overlayfs docker.io/library/alpine:latest
...
containerd [main]  ➜ sudo ctr snapshot ls
KEY PARENT KIND
# with this fix, default platform is used for pull and unpack.
containerd [ctr-pull-transfer-service]  ➜ sudo ctr i pull --snapshotter=overlayfs docker.io/library/alpine:latest
...
containerd [ctr-pull-transfer-service]  ➜ sudo ctr snapshot ls
KEY                                                                     PARENT KIND
sha256:75654b8eeebd3beae97271a102f57cdeb794cc91e442648544963a7e951e9558        Committed

[djdongjin]

hi @dmcgowan could you PTAL this PR when you get some time? thanks.

This makes --snapshotter take effect when no platform is passed (use default platform), when using transfer service.

[chrishenzie]

This is probably out-of-scope for this PR, but are there other subcommands using all-platforms that would benefit from this check?

[djdongjin]

Other commands just ignore --platforms if both are specified, e.g., in Fetch:

containerd/cmd/ctr/commands/content/fetch.go

Lines 137 to 143 in e514bae

	if !cliContext.Bool("all-platforms") {
	p := cliContext.StringSlice("platform")
	if len(p) == 0 {
	p = append(p, platforms.DefaultString())
	}
	config.Platforms = p
	}

I think error early is better than ignore one flag, but am okay to keep it the same with other commands

[djdongjin]

/cherrypick release/2.0

[k8s-infra-cherrypick-robot]

@djdongjin: new pull request created: #11139

In response to this:

/cherrypick release/2.0

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