bring back exiv2json (or remove the remnants from the repo)

[jpicht]

Is your feature request related to a problem?

The exiv2json sample has been removed by #2183, but is still referenced in the docs, vs2019 project, doxygen, and tests/suite.conf. As I'm using it (indirectly) in some scripts I'd like for it to come back.

It has been part of the exiv2 binary package for my linux distribution for quite a long time, so I never realized it wasn't in fact supported, but just a sample.

Describe the solution you would like

I'd like the merge be reverted and the problem with the aging JSON library be solved by porting to a newer json-library like (for example) nlohmann/json. As I'm familiar with that library, I would volunteer to do the work, if the MIT license is acceptable for you to include that as a dependency.

Describe alternatives you have considered

Providing a compatible tool, probably based on the last version, as a third-party package.