Rework the existing methods create_style and create_style_from_file to a more generic method capable of posting a style as XML, SLD or zip. According to the doc it should be possible to post the style definition as JSON as well, but we couldn't make it work so far so we probably won't implement it.

Regarding SLD, we don't plan to create or manipulate them, but instead to read them from the file system (as currently implemented) or from the REST API (to copy an existing style). They will be passed as binary blobs so that serialization / deserialization is not needed.