[Dart] Implement response headers

[provokateurin]

Is your feature request related to a problem? Please describe.

The dart generator doesn't support response headers like described in https://swagger.io/docs/specification/describing-responses. You can get them manually using the WithHttpInfo methods, but it is very inconvenient to do it that way.

Describe the solution you'd like

Implement a way to get the headers from the responses with the existing methods and not using the WithHttpInfo methods.

[ahmednfwela]

@jaumard (2018/09) @josh-burton (2019/12) @amondnet (2019/12) @sbu-WBT (2020/12) @kuhnroyal (2020/12) @agilob (2020/12) @ahmednfwela (2021/08)