Add to_flat_index method to MultiIndex
- [X] closes #19950
- [X] tests added / passed
- [X] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

Very simple implementation at the moment. The thought here is to introduce this method and perhaps subsequently extend to allow for string concatenation of the elements. Longer term there could also be a keyword added to `.agg` of GroupBy which will dispatch to this instead of simply returning a MultiIndex column, which could alleviate some of the pain users are experience when trying to rename columns after an aggregation.

@TomAugspurger and @jorisvandenbossche from the dev chat today