Support streaming responses in zio.http.api

[jdegoes]

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

Streaming responses is supported in the api of zio.http.api, but is not supported in the implementation for client and server (APIClient.scala, APIServer.scala, located in /src/main/scala/zio/http/api/internal/).

Describe the solution you'd like

Streaming responses should be supported for both client and server, with accompanying test suites.

Describe alternatives you've considered

N/A

Additional context

See the existing implementation for how a single value is deserialized, and in particular, update both client and server to check the API#output sum type in order to determine whether the response is supposed to be a single value or a stream of elements.

[ithinkicancode]

I'm going to take a crack at this ticket. Please assign it to me. Thanks.

[jdegoes]

Clearing assignees for now.