Make regexp_match take scalar pattern and flag

[viirya]

Which issue does this PR close?

Closes #5246.

Rationale for this change

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.

What changes are included in this PR?

This patch changes regexp_match to take Datum as parameters. It allows scalar pattern and flag.

For two benchmarks running against same array but one using pattern array and other using pattern scalar, scalar run takes ~50% less time.

regexp                  time:   [6.5667 ms 6.5763 ms 6.5857 ms]
                        change: [-2.3573% -1.6335% -0.9522%] (p = 0.00 < 0.05)
                        Change within noise threshold.

regexp scalar           time:   [3.4878 ms 3.4925 ms 3.4973 ms]
                        change: [-3.4210% -3.1127% -2.7738%] (p = 0.00 < 0.05)
                        Performance has improved.

Are there any user-facing changes?

[tustvold]

Had a quick review, looks good mostly just some stylistic comments to avoid unwrap, I do think the method should take &dyn Array and not be a mix of generic and type-erased

[viirya]

Thank you @tustvold