[ArrowStringArray] PERF: use pyarrow.compute.utf8_length if available
```
[ 25.00%] ··· strings.Methods.time_len                                                                                                                    ok
[ 25.00%] ··· ============== ==========
                  dtype                
              -------------- ----------
                   str        19.1±0ms 
                  string      6.82±0ms 
               arrow_string   1.93±0ms 
              ============== ==========
```