Consider to extract Zserio wrapper types from detail namespace

[mikir]

The base type of the Zserio wrapper types is in the namespace detail. This is a problem if we want to do template metaprogramming. Like support a specific operator only if something is a zserio numeric type.

Example:

template <typename T, 
    std::enable_if_t<std::is_base_of_v<zserio::detail::NumericTypeWrapper<typename T::ValueType>, T>, int> = 0>
void PrintZserioNumericType(const T& t)
{
    std::cout << t.ValueType();
}

Consider to rename wrapper types removing 'wrapper' word as well.

Reported by @reinco.