[csharp-netcore] Enum parameter serialization to use EnumMember value when provided.

Created by: jafin

PR relates to #11447 (closed)

Change method ParameterToString (ClientUtils) to check if obj is Enum and has a EnumMember prop. If so use that value over the existing logic of obj.ToString()

This fixes an issue whereby the enum value does not match the value in the openApi schema.

This PR also modifies the location of the yaml location for csharp-netcore/OpenAPIClient-net5.0, to a cloned copy of the java petstore API with changes to trigger the scenario described here. This was done to avoid disturbing clients (java) unrelated to this change.

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