ENH: include conversion to nullable float in convert_dtypes()
xref https://github.com/pandas-dev/pandas/issues/38110

There is one potentially corner case: what with floats that are all "integer"-like? I think we want to keep returning nullable int for that, at least by default, and that is what I did now in this PR But we might want to add a parameter controlling that behaviour? (but that can also be added later on if there is demand for it)