String dtype: allow string dtype for non-raw apply with numba engine
https://github.com/pandas-dev/pandas/pull/55104 enabled using numba with `raw=False` in `apply()` (BTW, this seems rather undocumented?), but that PR had some specific hack for object dtype index. Extending that hack to also convert str dtype index to numpy strings under the hood (this might hurt performance, but for now just want to get this working)




xref https://github.com/pandas-dev/pandas/issues/54792
Thanks @jorisvandenbossche 