Support for arbitrary XML in ExtendedData

[sheromon]

First I want to say thank you for sharing fastkml. It's been very useful to me, and I appreciate your work.

Recently, I needed to make a KML file with arbitrary XML in the ExtendedData of Placemark objects, and I found that this feature is not supported. I put in a simple hack to make it work, which was to make an ExtendedData object by passing in a pre-constructed etree and storing it in the object, then having from_element return the ExtendedData element with the provided etree as a child. I'd like to add this feature using a nicer implementation, though. I suppose the thing to do would be to add a TypedData class and pass TypedData elements into a TypedExtendedData object?

[cleder]

I think this can be achieved similar to gx:CascadingStyle #311

[cleder]

https://developers.google.com/kml/documentation/extendeddata#adding-arbitrary-xml-data-to-a-feature