Allow to control sampling via an `rng` function argument

See [here](https://github.com/py-econometrics/pyfixest/issues/453) for the motivation. 

Todo: add a `rng` argument [here](https://github.com/py-econometrics/wildboottest/blob/9587f91df2ae583ad95d3ed7f5e917d6ea0f0dc7/wildboottest/wildboottest.py#L57). 