[[BUG][C][cpp-restsdk] fix missing Set.h

[mlebihan]

fix missing Set.h issue in Twitter OpenAPI JSON #9969

- Handling `std::set` in cpp-restdsk

[cpp-pistache-server] taking into account a remark on this issue about cpp-pistache-server and its set management

- Switching `std::vector` to `std::set` for openapi set type in cpp-pistache-server

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[wing328]

@mlebihan thanks for the PR.

can you please pm me via slack when you've time to discuss this change?

https://join.slack.com/t/openapi-generator/shared_invite/zt-12jxxd7p2-XUeQM~4pzsU9x~eGLQqX2g

[wing328]

merged #18675 to enhance the samples, clean up, etc