c8d/push: better output if single-platform is pushed from index

[laurazard]

Stacked on top of #5572

- What I did

Added a docker images --tree-style messaging to the "Not all multiplatform-content is present and only the available single-platform image was pushed" messaging during a docker push, to more clearly indicate explain what happened and which manifest was actually pushed.

- How I did it

This requires adding information about which candidates were considered for pushing to the aux stream – moby/moby#48782.

- How to verify it

• docker pull alpine
• docker pull --platform [other-platform] alpine
• docker tag alpine <my-namespace>/alpine
• docker push <my-namespace>/alpine

- Description for the changelog

- A picture of a cute animal (not mandatory but encouraged)

[laurazard]

This is a draft ⚠️ It needs cleaning up and I need to open a PR in moby to (discuss and) add the additional info to the aux output.