[cpp-pistache-server]: Fix build with pistache master branch

[sheabot]

• Fixes [BUG] [cpp-pistache-server] Generated cmake project does not build #9148
• Integrate pistache's meson build system into existing cmake build
• Upgrade build to C++17
• Replace Pistache::Optional with std::optional from C++17
• Disable running tests during build of nlohmann/json dependency
• Update petstore sample

CC: @ravinikam @stkrwork @etherealjoy @MartinDelille @muttleyxd

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[wing328]

@sheabot thanks for the PR. Is this a breaking change without fallback? Does the newly auto-generated C++ code support older version of C++?

[sheabot]

Hi @wing328. Pistache upgraded to C++17 in pistacheio/pistache#859, which makes C++17 the minimum requirement to build against pistache. Since then, the auto-generated C++ code doesn't actually build against the pistache master branch.

So to answer your question, yes this is a breaking change and no older versions of C++ are not supported. This is necessary due to pistache's breaking changes. Without this, the auto-generated code will forever be pinned to an old version of pistache before these changes were merged.

[wing328]

@sheabot thanks for the explanation. Let's move ahead with this change.