[cpprestsdk] fix string conversion, add integer enum support

[srgbtl]

Fixes #10301

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[wing328]

Tested locally with integer enum and the result is good

[rickyma]

Hey @srgbtl @wing328 . How do we use this feature? What should I put into all.yaml?

Could you please give me an example of how to define the enums in all.yaml? I've encountered the same parameterToString error when using the latest OpenAPI generator 7.12.0 cpprestsdk.

[guilhermewerner]

@rickyma I have the same problem, in the latest version.

[rickyma]

It should be fixed by #20836 in the next release. @guilhermewerner