Config visitor API with JSON implementation

[jayv]

Frequently requested feature is a JSON representation of a materialized FlexiConfig, this PR adds:

• a <concepts> based Reader::visit(..) API to walk the materialized config tree
• enable a dump(..) to a custom ostream
• a basic and pretty printing JSON visitor
• python bindings for Reader::dump() and Reader::json(pretty=true)
• tests C++ JSON visitor
• JSON is validated by the python JSON parser through the python bindings
• updated readme
• enable python testing from Docker
• fixed CMake Python integration, python test was not included in ctest and possibly not running in CI