[C++][Pistache] Serialize integer enums if possible

[CTerasa-ep]

In OpenAPI it is possible to define an enum schema containing integers only.

Similar to the following JSON snippet:

...
  "components": {
    "schemas": {
      "size": {
        "type": "integer",
        "description": "Container size",
        "enum": [
          10000,
          20000,
          100000,
          200000,
          300000,
          1000000,
          1200000,
          2500000,
          5000000,
          10000000
        ]
      }
    }
  }
...

To correctly serialize this we need to convert to JSON integers. We can achieve this by feeding nlohmann JSON objects directly with integers instead of strings.

For the C++ pistache server adapt the enum models to serialize integer values if possible.

C++ committee: @ravinikam, @stkrwork, @etherealjoy, @MartinDelille, @muttleyxd

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