[C++][pistache] string enum code generation improvement

Created by: mfyuce

• A class that has an anyOf specification, in cpp side will have no members: instead it should have a member having the type stringenumclassname_anyOf
• Thus, Its == operator is not present or wrongly filled
• A string enum, should have a better usage, hence the setEnumValue
• this PR, is a brigde between stringenumclassname_**anyOf** and stringenumclassname
• anyOf specification is not just about Enums, so a better handling is needed from mustache templates, hence new template model parameter isStringEnumContainer

(Continuing from #1317 (closed) )

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