Fix URL paths following recommendations by Igor/Shaun: follow suggestions in [11]

[11] Instead of using private static strings, consider using a separate (compile-time) constexpr std::arraystd::string_view in a nested namespace (e.g. Crest::Detail) or in the anonymous namespace of the .cxx file with enumeration for the indices. Access would be like, for example: current_path += endpoints[METHOD_IOVS];