[REQ] Implement generator for cpp-httplib client/server

Describe the solution you'd like

I would like to have a new generator in OpenAPI Generator that produces C++ client/server code using the cpp-httplib library. This generator should create code that leverages httplib for HTTP client and server functionality, making it easier to integrate OpenAPI-defined APIs with modern, lightweight C++ projects.
Also I would like to use nlohmann json for serialize and deserialize the models generated instead of boot property tree.

Describe alternatives you've considered

Using the existing C++ generators (such as C++ REST SDK or Pistache), but they do not support httplib and may have heavier dependencies or different design philosophies.
Manually writing the integration code for httplib based on the OpenAPI spec, which is time-consuming and error-prone.

Additional context

The issue names no implementation files or tests. Start by reviewing the existing C++ REST SDK and Pistache generators, then clarify the client/server scope and model serialization requirements; done means a new generator produces C++ code using cpp-httplib and nlohmann json.