introduce ability to define image reference as {name,tag,digest}

[ndeloof]

introduce ability to define a service image as a combination of name, tag and digest (aligned with https://github.com/distribution/reference terminology)

services:
  test:
    image:
      name: registry.com/foo
      tag: bar
      digest: sha256:1234...

to be debated:
Shall we also split name as registry+repository (see https://github.com/opencontainers/distribution-spec/blob/main/spec.md) ?

closes docker/compose#11743