Update provider internal fields logic

[webb-ben]

No description provided.

[C-Loftus]

It appears this was failing because of the fact that fields is a @property but this property does not have a setter defined. I fixed the geopandas provider. I did this just by bringing in the property definition into the same class and then creating a setter function. (Had to bring it over since I don't believe we can create a setter for a property that was inherited from the base class)

I am assuming this is the pattern you want but let me know if I misinterpreted

[webb-ben]

Yes that is because of geopython/pygeoapi#1727