Adds isInnerEnum to CodegenProperty

This is a part of a larger PR that is being broken up. Adds isInnerEnum so the enum may be used outside of the class, such as in a JsonConverter class. You can see how I will use it here.

https://github.com/OpenAPITools/openapi-generator/blob/cb2ad559315243a5329fb5e66820617e44e9e92d/modules/openapi-generator/src/main/resources/csharp-netcore/libraries/generichost/JsonConverter.mustache#L46

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