ASV: add benchmarks for concatenating and appending of CategoricalIndex (GH38149)

- [x] closes #38149
- [x] tests added / passed
- [x] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing.html#code-standards) for how to run them
- [x] whatsnew entry

thanks @avinashpancham 