Find nlohmann_json downstream if EDM4hep has support

[tmadlener]

BEGINRELEASENOTES

• Add an EDM4HEP_WITH_JSON cmake option (default ON) to be explicit about whether we want to build EDM4hep with JSON support or not.
  • Properly propagate this to downstream packages.
  • If set to OFF the edm4hep2json executable will not be built.

ENDRELEASENOTES

[tmadlener]

There is a potential discussion point: Should we just build EDM4hep with JSON support unconditionally? Currently we make that dependent on whether we find nlohmann_json, which is not very explicit, and I would actually prefer to be explicit about that e.g. via an option.

[andresailer]

There is a potential discussion point: Should we just build EDM4hep with JSON support unconditionally? Currently we make that dependent on whether we find nlohmann_json, which is not very explicit, and I would actually prefer to be explicit about that e.g. via an option.

👍

[andresailer]

Thanks for putting this in so quickly!