Start and finish streaming trace in impl metod

[rm-openai]

Closes #435 and closes #538.

Unit tests.

[JilinJL]

@rm-openai Is there any problem here, please viewit very much, 🌹🌹
#516

[pakrym-oai]

I might be wrong but won't this break the following use case:

    result = Runner.run_streamed(
        agent, "Write ten haikus about recursion in programming."
    )

    i = 0
    async for event in result.stream_events():
        with custom_span("Processing event " + str(i)):
            print(event)
            i += 1

[rm-openai]

Yup, you're right. Talked on slack, the fix for this would be to manually attach to the trace via:

  async for event in result.stream_events():
        with custom_span("Processing event " + str(i), parent=result.trace):
            print(event)
            i += 1