Added cpp httplib server codegen

[rajvesh]

Introduces:

• Code generation for cpp httplib server
• API and model namespace customization.
• Custom project naming
• Pure virtual functions with request model arguments for custom logic
• apiNamespace, modelNamespace, projectName as additionalProperties
• CMake library integration
• Auto-generated API model usage README

To close #21466

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 || exit
  ./bin/generate-samples.sh ./bin/configs/*.yaml || exit
  ./bin/utils/export_docs_generators.sh || exit
  

  (For Windows users, please run the script in WSL)
  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*.
  IMPORTANT: Do NOT purge/delete any folders/files (e.g. tests) when regenerating the samples as manually written tests may be removed.
• File the PR against the correct branch: master (upcoming 7.x.0 minor release - breaking changes with fallbacks), 8.0.x (breaking changes without fallbacks)
• If your PR solves a reported issue, reference it using GitHub's linking syntax (e.g., having "fixes #123" present in the PR description)
• If your PR is targeting a particular programming language, @mention the technical committee members, so they are more likely to review the pull request.

[rajvesh]

Hello @wing328,
Could you or anyone from technical committee can review and approve the workflow for this PR?
Thank you.

[wing328]

thanks for contributing a new generator

can you please a github workflow and samples generated from the new generator similar to what we did in https://github.com/OpenAPITools/openapi-generator/pull/21547/files ?

[rajvesh]

Hello @wing328,
I have updated the requested changes.
Commit ID: 654865b
I see some issue in this branch:
This commit does not belong to any branch on this repository, and may belong to a fork outside of the repository.
Please let me know if any further changes are required. Request you or team to review the changes.

Thank you.

[rajvesh]

Hello @wing328,
I have updated the requested changes.
Commit ID:178f1059a41275a0012ef2aa0b820a810fb6cfff
Please let me know if any further changes are required. Request you or team to review the changes.

Thank you.

[wing328]

can you please review the build failure when you've time?

[wing328]

why remove this?

[rajvesh]

I Will check. It is not intentional.

[wing328]

👌

please also merge the latest master into your branch when you've time.