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

[cleder]

@copilot
The simplest way to add user data to a Feature is to add the data directly as a value of <ExtendedData>. Google Earth preserves this data but does not process it.

Custom data elements added in this way need to include a namespace prefix (xmlns:prefix="namespace"). This prefix can be added to the <kml> element or to the <ExtendedData> element. The external namespace prefix must qualify each instance of user data, as shown in the following example:

<kml xmlns="http://www.opengis.net/kml/2.2">
<Document>
  <name>ExtendedData Test</name>
  <Placemark>
    <name>CampsiteData</name>
    <!-- Imported schema requires use of namespace prefix -->
    <ExtendedData xmlns:camp="http://campsites.com">
      <camp:number>14</camp:number>
      <camp:parkingSpaces>2</camp:parkingSpaces>
      <camp:tentSites>4</camp:tentSites>
    </ExtendedData>
    <Point>
      <coordinates>-114.041,53.7199</coordinates>
    </Point>
  </Placemark>
</Document>
</kml>

example at tests/ogc_conformance/data/kml/custom/Data-Ok.xml

Create documentation and example how to achieve this like in docs/working_with_kml.rst and examples/transform_cascading_style.py