toQueryMap method
Returns a single-entry map suitable for merging into
Connector.defaultQuery().
This method exists because Dio's Options object does not support query
parameters — they must be supplied at the request-building stage through
the connector's default query map. Call this in your connector's
defaultQuery() override to have Lucky append the API key to every
outgoing request URL.
Implementation
Map<String, String> toQueryMap() => {key: value};