oci-multiarch: fix MC defaults, deploy dependency, and OCI layout

Three issues prevented oci-multiarch.bbclass from producing usable
multi-architecture container images:

1. MC defaults pointed to vruntime-* multiconfigs, whose BBMASK blocks
   OCI tooling. Changed to container-* multiconfigs which use the new
   vcontainer distro without BBMASK.

2. mcdepends targeted do_image_oci, but the OCI output is only deployed
   to deploy/images/ by the later do_image_complete task. The bbclass
   then failed to find the OCI directory at the expected deploy path.
   The original implementation assumed do_image_oci was the final step,
   but OE-core's image pipeline has a separate deploy phase.

3. The OCI Image Index was written directly into index.json with
   multiple manifest entries. This is valid per the OCI spec but skopeo
   requires index.json to reference a single entry when there are
   multiple images. The fix writes the multi-platform image index as a
   blob in blobs/sha256/ and has index.json reference it with a single
   entry of mediaType application/vnd.oci.image.index.v1+json. This
   nested layout is what tools like buildah and crane produce for
   multi-arch images, and is required for 'skopeo copy --all' to work.

Also adds container-base-multiarch.bb recipe that wires up container-base
for aarch64 + x86_64 builds via the oci-multiarch class.

Signed-off-by: Bruce Ashfield <bruce.ashfield@gmail.com>

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