Refactor CI copy-to-production in a reusable composite action

[NiccoloFei]

The copy-to-production steps that are being done in bake_targets.yml are a generic way to copy and sign images from a testing registry to a production registry.

These set of steps work under the assumptions that the source testing registry is always named <my-production-registry>-testing,
so the code can just determine the proper destination registry by stripping off the -testing suffix (which we can make configurable in the action).

We could refactor these steps in a reusable composite GH action, to allow reusing the same code as part of all the other "Bake" repositories. The only input required should be the output metadata generated by the bake-action.