How can I provide an electrical outlet at my portable kitchen island?
There are pendants and there are pendants. An item like this Power Pendant Outlet Drop is designed for the purpose you describe. An ordinary residential grade hanging light is NOT. There are a few possible issues:


An ordinary hanging light fixture is designed to carry the weight of the fixture. It is not designed to handle frequent manipulation and the stress of daily plugging/unplugging of appliances.
An ordinary light fixture (hanging or a table lamp or whatever) will often have smaller size wire than needed for a full 15A or 20A circuit because the requirements (and associated UL testing/rating) are based on the fixture having a maximum continuous usage far lower than 15A or 20A. For example, a 6-bulb chandelier might be rated for 6 x 60W = 360W == 3A continuous usage. Therefore, it can have 18 AWG wires from the bulbs on up to the ceiling box. You do NOT want to run your toaster oven on 18 AWG wires.
Grounding - I do not know if ANY edison-base to receptacle adapters are available with proper 3-wire grounded receptacles. I know the ones I Google'd now (UL listed, name brand) are NOT grounded, and I suspect that by design it is not possible to have such an adapter properly grounded.
GFCI - If the lighting circuit is on a GFCI breaker (or downstream "Load" of a GFCI receptacle) then you are protected (and this helps mitigate the grounding concern). But if that is not the case then you would need to retrofit a GFCI breaker. If you can't do that (e.g., old fuse box or double-stuff breakers) then you are going against modern code for kitchen receptacles.


In short: This is not a good idea unless you are prepared to install a "real" pendant power source.

As far as the alternative of a floor outlet - that can easily solve most of these issues. However, a floor outlet will be too far away for modern kitchen appliances that typically have (based on modern kitchen electrical code) ~ 2 - 3 ft. cords. A hanging pendant (of the right type) does not have that problem.