[C++] Kernels with scalar input

[Uwe L. Korn]

Hello all,

I'm trying to implement a `contains` kernel that takes as an input a 
StringArray and a scalar string (see 
https://issues.apache.org/jira/browse/ARROW-9160). I feel confident with the 
rest of the new Kernels setup but I didn't find an example kernel where we also 
pass in a scalar attribute. Can someone point me to an approach on how to do 
this?

Best
Uwe