Function to_extension 

pub fn to_extension<'a>(
    fbb: &mut FlatBufferBuilder<'a>,
    name: &str,
    url: &str,
    version: &str,
) -> WIPOffset<Extension<'a>>

Writes one entry of the CityJSON extensions member into the header.

Only the reference is written — the name, the URL of the extension schema and the version, which is all extensions contains. The schema document itself is not fetched: doing so made writing a file depend on the network (and on the extension host still being up), and nothing in this crate reads the embedded schema back.