[RDF] Bring back a removed snapshot method

As discussed on the ROOT Users workshop, entirely removing interfaces
after one single ROOT release leaves users with broken code without
any hints on how to solve the problem. Instead, a minimal placeholder
should remain, which gives directions on how to proceed.
In that spirit, the templated snapshot overload is brought back and
marked deprecated.

This partly reverts commit 4da10a2.

Author: hageboeck

tree/dataframe/inc/ROOT/RDF/RInterface.hxx

@@ -1242,6 +1242,15 @@ public:
       return newInterface;
    }
 
+   template <typename... ColumnTypes>
+   [[deprecated("Snapshot is not any more a template. You can safely remove the template parameters.")]]
+   RResultPtr<RInterface<RLoopManager>>
+   Snapshot(std::string_view treename, std::string_view filename, const ColumnNames_t &columnList,
+            const RSnapshotOptions &options = RSnapshotOptions())
+   {
+      return Snapshot(treename, filename, columnList, options);
+   }
+
    ////////////////////////////////////////////////////////////////////////////
    /// \brief Save selected columns to disk, in a new TTree or RNTuple `treename` in file `filename`.
    /// \param[in] treename The name of the output TTree or RNTuple.