Support Server Sent Event

[hurelhuyag]

Is your feature request related to a problem? Please describe.
The Server Sent Event (SSE) is an essential feature for modern applications and We proved SSE is possible with Dio.

Describe the solution you'd like

@GET("/sse-endpoint")
Stream<SseMessage> sse();

or maybe we should introduce @SSE annotation instead of @GET.

Describe alternatives you've considered
Currently, I'm using Dio directly for this kind of connection. But It can generated by retrofit_generator

Additional context
cfug/dio#1279

Also, I created a transformer to build SseMessage. cfug/dio#1279 (comment)