[RDF] Fix inconsistency in RDF::Snapshot interfaces for jitted calls

now Snapshot accepts to identify columns:
- A regex as a string_view
- A vector<string>
- A initializer_list<string>

This allows users to invoke it as follows:
1. tdf.Snapshot("t", filename, "an*");
2. tdf.Snapshot("t", filename, {"ans"});
3. tdf.Snapshot("t", filename, {{"ans"}});