add soci to nerdctl image convert

[coderbirju]

This PR adds functionality to support nerdctl image convert using soci.
This follows the new soci convert feature.

cc: @Shubhranshu153 @swagatbora90 @AkihiroSuda

[Shubhranshu153]

@AkihiroSuda Can we add this to milestone 2.1.3 as well. Hopefully we can get it through tomorrow.

[Shubhranshu153]

Needs a rebase to fix existing soci tests.

[Shubhranshu153]

Reduce the min size of the image or use larger image size to fix the test.

| 🟠 time="2025-07-01T19:09:49Z" level=info msg="Converting image from ghcr.io/stargz-containers/a |
        |         | lpine:3.13-org to docker.io/library/testimageconvert-soci-converted-image-aceae1d3:latest using  |
        |         | SOCI format"                                                                                     |
        |         | time="2025-07-01T19:09:49Z" level=info msg="soci: ztoc skipped - layer sha256:ca3cd42a7c9525f6ce |
        |         | 3d64c1a70982613a8235f0cc057ec9244052921853ef15 (application/vnd.docker.image.rootfs.diff.tar.gzi |
        |         | p) size 2811947 is less than min-layer-size 20971520"                                            |
        |         | time="2025-07-01T19:09:49Z" level=info msg="soci: soci: no ztocs created, all layers either skip |
        |         | ped or produced errors"                                                                          |
        |         | time="2025-07-01T19:09:49Z" level=fatal msg="failed to convert image to SOCI format: exit status |
        |         |  1"             

[AkihiroSuda]

Thanks