Have describe command display digests of image layers

[alexgaganashvili]

go run cmd/imgpkg/imgpkg.go describe -b projects-stg.registry.vmware.com/tce/cert-manager@sha256:e321a8936343a3eb05676aa2cf5ba7b5a07a8867e94cab75bff68800fa6b0459 -o yaml

sha: sha256:e321a8936343a3eb05676aa2cf5ba7b5a07a8867e94cab75bff68800fa6b0459
content:
  images:
    sha256:4ab2982a220e1c719473d52d8463508422ab26e92664732bfc4d96b538af6b8a:
      annotations:
        kbld.carvel.dev/id: quay.io/jetstack/cert-manager-webhook:v1.9.1
        kbld.carvel.dev/origins: |
          - resolved:
              tag: v1.9.1
              url: quay.io/jetstack/cert-manager-webhook:v1.9.1
      image: quay.io/jetstack/cert-manager-webhook@sha256:4ab2982a220e1c719473d52d8463508422ab26e92664732bfc4d96b538af6b8a
      imageType: Image
      layers:
      - digest: sha256:b9f88661235d25835ef747dab426861d51c4e9923b92623d422d7ac58eb123e9
      - digest: sha256:acfb93fdd86951555e2f45fb3fb417b12ef444d4fb46c94121121c8c0ef57e75
      - digest: sha256:3037f5ce7599ba6fb57a17ff4aefca8cbbeb2a2ecc8363b24fdedc1fca74ccb7
      - digest: sha256:02efe101fd3a9234df2c84bfa72a7b1b71babdbc9ab16f6df7fada1487433f20
      origin: quay.io/jetstack/cert-manager-webhook@sha256:4ab2982a220e1c719473d52d8463508422ab26e92664732bfc4d96b538af6b8a
...
...

[joaopapereira]

The PR #622 supersedes this one.