CMake: Install toml11 and nlohmann_json for inclusion in downstream code

[franzpoeschel]

This is an attempt at finding a solution for this issue ComputationalRadiationPhysics/picongpu#5355

Problem: If a downstream code wants to use nlohmann_json / toml11 as well, then it must use the same version used also in openPMD, otherwise it might happen that the same symbol defined by toml11/nlohmann_json might exist in different implementations (different versions) in libopenPMD.so and the compiled binary of the downstream. The linker might then jump between implementations, causing undefined behavior.

Attempted solution in this PR:

1. By default, do not use internally shipped dependencies, but look for an installation of toml11/nlohmann_json in the system. (not yet implemented)
2. If using an internally shipped dependency, then install it along with openPMD such that these versions become available in the software environment.
3. Downstream codes can then use the same logic to agree on common versions of toml11/nlohmann_json.

Alternatives:

• Maybe include toml11 / nlohmann_json into their own namespaces in openPMD instead?

TODO:

• Look for nlohmann_json/toml11 in the system software environment by default
• Implement this logic also for workflows using openPMD_toml11_src / openPMD_json_src
• Add a flag to opt-out from this. (Yes, opt-out, this can only be a viable solution for the described issue if we can assume that an openPMD installation will share its versions of toml11 / nlohmann_json by default)
• Open PRs at toml11/nlohmann_json for binary symbol versioning with inline namespace

[psychocoderHPC]

@franzpoeschel Please take care that dependency fetching from the web is on some systems not possible due to restrictions that login nodes or compute nodes are not allowed to download things from the web.
@chillenzer has experience with that because he solved the issue already in https://github.com/alpaka-group/mallocMC

[franzpoeschel]

@franzpoeschel Please take care that dependency fetching from the web is on some systems not possible due to restrictions that login nodes or compute nodes are not allowed to download things from the web. @chillenzer has experience with that because he solved the issue already in https://github.com/alpaka-group/mallocMC

Dependency fetching is already part of our build system, this PR does not change that, but rather adds an install step. Offline builds are fully supported (and we use them for distribution via package managers), see CMake variables:

openPMD_USE_INTERNAL_TOML11
openPMD_USE_INTERNAL_JSON
openPMD_toml11_src
openPMD_json_src