WIP: daemon/c8d: Image layer

[vvoland]

• stacked on: daemon/export: Stop when context is canceled  #49265

Implement image-based readonly layers.

Target use case: #48798

This needs some further work (probably in follow-up):

• Combine with the code image_builder.go that does a very similar thing for the classic builder
• Move top layer package to a graphdriver store internals and reshape it into a simpler interface used between the daemon and image service