[cpp-pistache] Allow all Apis under a single endpoint to be hosted in one server

[etherealjoy]

PR checklist

• Read the contribution guidelines.
• Ran the shell script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh and ./bin/security/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates). Windows batch files can be found in .\bin\windows\.
• Filed the PR against the correct branch: master, 3.4.x, 4.0.x. Default: master.
• Copied the technical committee to review the pull request if your PR is targeting a particular programming language.

Description of the PR

• Allow all APIs in the same endpoint to be hosted in a single server
• Add include of json.hpp when no models are imported
• small changes to simplify CMakeLists.txt and to be able to exit the server with a Ctrl+C

@stkrwork @MartinDelille @ravinikam @fvarose
Fixes #1168
Fixes #548

[CyrilleBenard]

I just checked the new code generation that now allows to embed all the API under one HTTP endpoint and that does exactly what expected. Really nice
Consequently it solves the question asked in #548
Really a nice job 👍

EDIT: fixes also #1168

[etherealjoy]

@stkrwork
Thank you for the review.

[wing328]

@etherealjoy thanks for the PR, which is included in the v3.3.2 release: https://twitter.com/oas_generator/status/1057649626101112832