Add ZIO streaming support for chat completions

[DybekK]

This PR adds streaming support for Create chat completion endpoint using ZIO as a streams implementation.

Here are the main changes made in this PR:

• out-of-the-box streaming support has been added using the ZIO library by extending current OpenAI class using implicit class:

  
  package sttp.openai.streaming

package object zio { implicit class extension(val client: OpenAI) { def createStreamedChatCompletion(chatBody: ChatBody): StreamRequest[Either[OpenAIException, Stream[Throwable, ChatChunkResponse]], ZioStreams] } }

- integration tests have been added using HttpClientZioBackend.stub