refactor(streaming): move to an iterator pattern

[RobertCraigie]

Now instead of requiring you to define a subclass and pass it as an argument, we yield special SDK constructed events, e.g.

from anthropic import AsyncAnthropic

client = AsyncAnthropic()

async with client.messages.stream(
    max_tokens=1024,
    messages=[
        {
            "role": "user",
            "content": "Say hello there!",
        }
    ],
    model="claude-3-opus-20240229",
) as stream:
    async for event in stream:
        if event.type == "text":
            print(event.text, end="", flush=True)
        elif event.type == 'content_block_stop':
            print('\n\ncontent block finished accumulating:', event.content_block)

    print()