Make regexp_match take scalar pattern and flag

[viirya]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.

regexp_match kernel currently assumes patterns and flags are all arrays with same length as string array. For the kernel, one usual case is to use scalar pattern and flag, we don't need to build pattern hash map.

Describe the solution you'd like

Make regexp_match kernel take scalar pattern and flag

Describe alternatives you've considered

Additional context

[tustvold]

label_issue.py automatically added labels {'arrow'} from #5245