dnsdist: Expose toString of various objects to Lua

[zeha]

Short description

For better introspection all creatable objects should have toString() methods. Actually many objects already have toString() implemented in C++, but then it's not hooked up to Lua.
This adds a few (easy) objects to Lua, obviously more would be better.

Checklist

I have:

• read the CONTRIBUTING.md document
• compiled this code
• tested this code
• included documentation (including possible behaviour changes)
• documented the code
• added or modified regression test(s)
• added or modified unit test(s)