Add the Key4hep Stack

• Add all the packages of the Key4hep stack. I propose for all of them to be lower-case for simplicity, then the name is expected and one doesn't have to remember which ones are capitalized or not as it happens with other packages in the LCG stacks.
• Add a heptools-devkey toolchain with the latest tagged version of the packages that are needed for the Key4hep stack

Needs:

• !2666 (merged) to build Gaudi on MacOS
• !2640 (merged) to build DDFastShowerML, otherwise finding openmpi at the CMake stage fails even though openmpi is there.
• https://github.com/HEP-FCC/dual-readout/pull/44 to build dual-readout on MacOS.

Compilation tested on Alma 9 with GCC 14 and Clang 19 (most of the packages, as devkey doesn't fully build because of packages outside of the Key4hep stack), and MacOS 15. Other changes, like documentation for LCGinfo and setup scripts will come later.