cpp-common: add nlohmann/json header-only dependency

cpp-common: add nlohmann/json header-only dependency

This commit adds the C++ nlohmann JSON library which comes from the
MIT-licensed <https://github.com/nlohmann/json/> project [1].

Babeltrace 2 will be using this library to encode JSON strings during
the generation of CTF 2 traces.

In any Babeltrace 2 C++ file, include it like this:

    #include "cpp-common/nlohmann/json.hpp"

[1] https://raw.githubusercontent.com/nlohmann/json/a0c1318830519eac027a31edec1a99ce1ae5670e/single_include/nlohmann/json.hpp

Signed-off-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Change-Id: I139ea0c0763a129a8d4db351f8ece2edb1c112dd
Reviewed-on: https://review.lttng.org/c/babeltrace/+/7321
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/7138
Tested-by: jenkins <jenkins@lttng.org>