Allow using system provided json library

[mattiaverga]

Some Linux distributions have their own Niels Lohmann's json library packaged, this will add an option to use the system provided one (default to use bundled library).

[mattiaverga]

I think I should also make indi provide a HAVE_BUNDLED_JSON flag, so indi-3rdparty targets requiring json.h know that they should require system's json library... but I'm not expert in cmake and didn't found a way to do that, yet.

[knro]

Can you please resolve the conflicts above?

[mattiaverga]

Rebase done.

About indi-3rdparty, currently both project use the INDI_SYSTEM_JSONLIB option manually.
I think I could have 3rdparty to check for the indi provided json library in FindINDI.cmake and require the system library if indilib was built with system json, but I think I would need to change the indi provided json header file name to distinguish from the upstream one. AFAIK, using find_path() will always check in system's paths, so using the same name for indi provided and upstream header file will not distinguish the two...

[knro]

I think using a different header name is OK if this makes it work across indi-core and indi-3rdparty reliably in both scenarios.

[mattiaverga]

I will update indilib/indi-3rdparty#861 ASAP

[mattiaverga]

Do we also want to add a workflow to test build using system libraries (json and http)?