[WIP] Add a bundle package and environment to build a basic "externals" stack

[tmadlener]

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This is a proposal to add a BundlePackage and an environment that can be used to build a minimal external base on top of which most (all?) Key4hep packages can be built. The current dependencies are mainly collected from trying to build Gaudi and podio on top of it, there are almost certainly packages missing for other "low level" Key4hep packages (e.g. I haven't checked DD4hep).

One of the potential use cases for this is container building where a basis container can be built up-front and only has to change infrequently, while nightly builds or release build without changes to this basis container can be layered on top of it. We (well mostly @madbaron) have been exercising this workflow a bit for building containers for the muon collider, and so far it works out quite well.

I am opening this PR for a general discussion to see what others think and also to see which packages should still go into this package.

The naming is also still up for debate, currently its base but it could just as well be external or minimal if we want to reserve base for something that already has the main building blocks of Key4hep as well (e.g. k4FWCore, DD4hep, EDM4hep, maybe k4geo).

[madbaron]

Just one note to keep in mind if we end up changing the packages included in "base": to keep the multi-step image build working on the github runners, we should keep the compilation time (including setup, checkout and upload to the registry) of each image/layer below 6 hours.

The current set takes about 5h 40m (see for example https://github.com/madbaron/mucoll-spack/actions/runs/12018805366)