ENH: add IntervalIndex.get_loc_exact to look for exact matches only
- [x] closes #19349
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

This PR add a 'exact' option to the parameter ``method`` on ``IntervalIndex.get_loc`` for accepting exact matches only.

In addition to it's direct advantage, it will later make it possible to make a PR to allow ``IntervalIndex.get_indexer`` calls with the other index having non-Interval values, and hence allow mixing mix Interval with non-intervals in ``pd.Index(..., dtype=object)`` instances. This would fix the remaining issue with #19021.