Add support for foreign members to FeatureWriter.

[dabreegster]

• [X] I agree to follow the project's code of conduct.
• [X] I added an entry to CHANGES.md if knowledge of this change could be valuable to users.

[dabreegster]

Successfully using this in https://github.com/dabreegster/routing-engines/blob/c58cc43832714c3c2ca17105f184b2885313eddd/aggregate_routes/src/osm2network.rs#L350

[michaelkirk]

Hrmm... my inline comment got lost in the ether.

What would you think about changing the API slightly to avoid building up the intermediate JsonObject just to pass it in?

pub fn write_foreign_member(&mut self, key: &str, foreign_member: &JsonValue) -> Result<()>;

[dabreegster]

What would you think about changing the API slightly to avoid building up the intermediate JsonObject just to pass it in?

Good idea, done. It was an odd API before to only allow one of the calls. Writing one key at a time is much nicer.

[dabreegster]

This gets a little nicer to use if we allow the same value types as serde_json::to_writer.

Good point, thanks! Done