Method `get_string()` should return a namespace leading by separator character

[IzabellaRaulin]

Method get_string() should return a namespace leading by separator to keep parity with snap-plugin-lib-go and snap-plugin-lib-py

For metric ["intel", "mock", "metric"] with "/" as a separator, executing mt.ns().get_string() returns:

Current behaviour	Expected behaviour
intel/mock/metric	/intel/mock/metric

Why this is important

To avoid adding leading separator in plugin, for example how it happens in pull request to rdt collector

Code in snap-plugin-lib-cpp:
No leading separator added

const std::string Namespace::get_string() const {
    std::stringstream ss;
    int i = 1;
    for (const auto& node : namespace_elements) {
        if (i < namespace_elements.size()) ss << node.get_value() << "/";
        if (i == namespace_elements.size()) ss << node.get_value();
        i++;
    }
    return ss.str();
}

Code in snap-plugin-lib-go:

func (n Namespace) String() string {
	ns := n.Strings()
	s := n.getSeparator()
	return s + strings.Join(ns, s)
}

Code in snap-plugin-lib-py:

[...]
        result = separator
        for n in self:
            result += n.value + separator
        return result[:-1]