[C++][Pistache] Generate API generalization interface

[CTerasa-ep]

The code generated for main() contains self-similar construction of, and calls to API implementation classes.

Example:

    PetApiImpl PetApiserver(router);
    PetApiserver.init();
    StoreApiImpl StoreApiserver(router);
    StoreApiserver.init();
    UserApiImpl UserApiserver(router);
    UserApiserver.init();

This sceams for OO generalization.

We could then store all *server objects inside one vector of API implementations and call init() on all of them.

    auto apiImpls = std::vector<std::shared_ptr<ApiBase>>();

    apiImpls.push_back(std::make_shared<PetApiImpl>(router));
    apiImpls.push_back(std::make_shared<StoreApiImpl>(router));
    apiImpls.push_back(std::make_shared<UserApiImpl>(router));

    for (auto api : apiImpls) {
        api->init();
    }

This patch set refactors some Pistache generator code and introduces a base class for APIs called ApiBase and adds tehe above construction code.

Additionally do some refactoring work in the Pistache generator code, that leverage some functional stream() methods that may make some intentions more explicit.

Technical Committee C++:

• @ravinikam
• @stkrwork
• @etherealjoy
• @MartinDelille
• @muttleyxd

PR checklist

• Read the contribution guidelines.
• Pull Request title clearly describes the work in the pull request and Pull Request description provides details about how to validate the work. Missing information here may result in delayed response from the community.
• Run the following to build the project and update samples:

  ./mvnw clean package 
  ./bin/generate-samples.sh
  ./bin/utils/export_docs_generators.sh
  

  Commit all changed files.
  This is important, as CI jobs will verify all generator outputs of your HEAD commit as it would merge with master.
  These must match the expectations made by your contribution.
  You may regenerate an individual generator by passing the relevant config(s) as an argument to the script, for example ./bin/generate-samples.sh bin/configs/java*.
  For Windows users, please run the script in Git BASH.
• In case you are adding a new generator, run the following additional script :

  ./bin/utils/ensure-up-to-date.sh
  

  Commit all changed files.
• File the PR against the correct branch: master (6.3.0) (minor release - breaking changes with fallbacks), 7.0.x (breaking changes without fallbacks)
• If your PR is targeting a particular programming language, @mention the technical committee members, so they are more likely to review the pull request.

[muttleyxd]

C++ and template code looks good to me

[CTerasa-ep]

Great! Thank you for the feedback and your effort!