bsf oci customise layer spreadout

[dr-housemd]

bsf could allow users to customise how OS package -> OCI layer mapping should look like i.e

• we could let user have 1 layer per OS package(ideal, should likely be the default)
• let user merge all dev packages into one layer and runtime into another.
• let user merge two or more OS packages to a single layer and remaining OS packages can be dedicated layers.

This allows user to make efficient use of network bandwidth, in case they've the packages in nix store already.
It also significantly improves patching time of images.