Fix use of vendored json when external is chosen

[gdt]

• zero AI use!
• Closes vendored nlohmann/json is used even if external was chosen #4781
• Tests added (there is no new code to test; one could essentially add the NOMERGE: commit as an rm in a CI run)
• Added clear title that can be used to generate release notes
• No new APis; this is a bug fix

nlohmann/json can either be a normal dependency ("external", the standard approach) or a vendored copy (for those who don't have it installed). There is a header that switches on EXTERNAL_NLOHMANN_JSON to use the system version instead of the vendored version. The build system fails to pass it to some compilations that use json.

This PR, minus the first commit which serves as a test, adds those defines.

An alternative change in the same spirit would be to add the define to all builds, which would be more robust than adding it to builds that use json today. A further change that would improve robustness would be to flip the define to VENDORED_NLOHMANN_JSON and thus if there are new targets that use json with passing it, builds in environments lacking system nlohmann/json will fail, instead of silently using the old code.

It will be interesting to see how CI goes for this branch, which has (temporarily for testing) removed the vendored code. The plan would be to rebase to drop that commit before merging.

[gdt]

On my system, with normal-system-dependency nlohmann/json, this branch builds and passes tests as well as master does (one issue in test_network already reported on the list), providing confidence that now all compilations using the json implementation switcher have the flag passed.