ci, add container matrix for serial and MPI HDF5 milestone versions

[steven-varga]

Goal

Revamp h5cpp container validation so local Podman/Docker can build and run h5cpp against serial and MPI HDF5 milestone versions.

Scope

• Add reusable Podman/Docker-compatible container files for HDF5 milestone versions.
• Cover serial and MPI flavors for:
  • HDF5 1.10.x
  • HDF5 1.12.x
  • HDF5 1.14.x
  • HDF5 2.x experimental
• Add runner scripts that select version and flavor.
• Default to podman locally while allowing CONTAINER_ENGINE=docker.
• Preserve existing Docker/MPI smoke-test behavior or provide compatibility wrappers.
• Ensure the 1.12 serial lane can be consumed by the HDF5 reference compatibility work.

Current reference-validation need

The current H5R reference compatibility work only needs the serial HDF5 1.12 lane now. MPI, 1.14, and 2.x are matrix infrastructure, not blockers for that feature.

Non-goals

• Do not require Spack as the developer contract.
• Do not make HDF5 2.x a required passing lane yet.
• Do not change h5cpp C++ source behavior.