Missing co-simulation support for FMUs with C++ runtime

[sturmk]

Description

OpenModelica is able to generate model exchange - FMUs with the cpp backend. However, co-simulation is currently unsupported. It would be nice to use co-simulation FMUs with the C++ backend. (A fist inspection of the generated C++ code reveals that it might be a work-around for issue https://github.com/OpenModelica/OpenModelica/issues/10790 .)