add visitor pattern implementation for Poco::Dynamic::Var

[bas524]

Complimentary for #2291
It's a simple implementation of runtime visitor
Main idea is using unique handler for every type which can be saved in the Dynamic::Var

As handler you can use lambda, std::function and function pointer with signature:

std::function<void(const T&)>;
void (*)(const T &);

example

// ...
Visitor visitor;
visitor.addHandler<std::string>([](const std::string& s) { std::cout << s; });
// ...
Poco::Dynamic::Var v = getVar();
if (visitor.visit(v))
{
    // successfully processed
}

[aleks-f]

@bas524 we need that CI passing. rebase your branch with devel to get the most recent changes and I will restart any failing CI (sometimes they just timeout) that does not show a serious bug

[bas524]

@bas524 we need that CI passing. rebase your branch with devel to get the most recent changes and I will restart any failing CI (sometimes they just timeout) that does not show a serious bug

Hi! I did it, but ci has failed and I think that my PR is not a reason