ENH: Add max_results kwarg to read_gbq
Adds support for the new `max_results` kwarg from pandas-gbq (added in [0.12.0](https://github.com/pydata/pandas-gbq/releases/tag/0.12.0)). Since `max_results` is a new kwarg, it is handled and tested in the same way as the `use_bqstorage_api` kwarg to maintain backwards compatibility.

❓ **Open question:** Setting `max_results=0` causes `pandas_gbq.read_gbq` to return `None` instead of a DataFrame. I've kept this behaviour the same in this PR, but maintainers may prefer to always return a (empty) DataFrame instead of `None`.

- [x] closes #34639
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry


@tswast  thanks @JohnPaton 