Replace fwd decl of json with wrapper type

[hahnjo]

Since nlohmann/json#3590, the basic_json class and the json using-declaration are located in a "versioned, ABI-tagged inline namespace". This makes it impossible to forward declare the type in REveElement.hxx.
Instead introduce a new struct REveJsonWrapper that just wraps a json object (after including the full nlohmann/json.hpp). As the struct is under our control, we can easily forward declare the type and use it for method arguments.

Fixes #11130

[phsft-bot]

Starting build on ROOT-debian10-i386/soversion, ROOT-performance-centos8-multicore/cxx17, ROOT-ubuntu18.04/nortcxxmod, ROOT-ubuntu2004/python3, mac1015/cxx17, mac11/cxx14, windows10/cxx14
How to customize builds

[hahnjo]

Two issues with this approach:

1. It's not fully API compatible with the old argument types (cannot bind to references of a different type)
2. Users cannot overwrite the functions anymore; if that's needed, we can make REveJsonWrapper.hxx public while keeping it out of the module (?).

[linev]

Problem is not only eve7 - where derived classes used in examples.

nlohmann/json used in TBufferJSON and planned for usage with TJSONFile.
There much more operations used with it.

Idea to wait for llvm13 where one fully can avoid forward declarations and use directly nlohmann::json

[hahnjo]

@linev no, usage of nlohmann/json in source / implementation files is completely fine. The only problem is with its usage in headers -> modules. Regarding llvm13, I still don't understand; see my comment #11130 (comment)

[hahnjo]

Not the right approach as we might want to have APIs that take nlohmann::json arguments; #11130 provides a fix for the current problems with versions 3.11+