FR: implement oci_index to support multi arch images

[thesayyn]

An example build file looks like below

# this image will have 4 layers in total
oci_image(
    name = "debian10",
    layers = [
        ":debian10_fs",
        ":debian10_passwd",
        ":libs"
    ]
)

platforms = {
    # 1:2+ transitions
    "@platforms//linux": [
        "@platforms//cpu:x86_64",    
        "@platforms//cpu:arm64",
        "@platforms//cpu:armel",
        "@platforms//cpu:ppc",
    ]
}

# will yield multi-platform image
oci_index(
    name = "debian_multi",
    image = ":debian10",
    platforms = platforms
)

[thesayyn]

PS: discovered that project-zot/zot#640 is a blocker