Defines response structs from a field list.
use ConvergerClient.Model,
fields: [conversation_id: "conversationId", token: "token", from: {"from", ConvergerClient.ChannelAccount}]Each field maps a struct key to a wire key, optionally with a nested model
(applied to a map, or to every element of a list). Unknown wire keys are
kept in :extra, so fields the server adds later are not lost.
The models are hand-written today. Once the OpenAPI document of #45 ships
at priv/static/openapi.json, they are checked against it (see the README).