flesh out types from the oci_ref data source

[imjasonh]

Fixes #1

This enables things like:

data "oci_ref" "alpine" { ref = "alpine" }

output "amd64" { value = oci_ref.alpine.images["linux/amd64"].image_ref }

or

# Sign all the images in the index.
resource "cosign_sign" "images" {
  for_each = oci_ref.alpine.images
  image = each.value.image_ref
}

# Verify all images in the index pass the policy.
data "cosign_verify" "images" {
  for_each = oci_ref.alpine.images
  image = each.value.image_ref
  policy = "..."
}

It also gives some useful data from an image's config, and all the data available in the manifest, in case that's useful.

output "created_at" {
  value = oci_ref.alpine.config.created_at
}