BUG: Raise ValueError for non numerical join columns in merge_asof
- [x] closes #29130
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

I checked for dtype object. Normally, I would try to check for allowed dtypes, but that would require a lot of dtype checks instead of only one. I hope object dtype is sufficient to exclude