feat: add generator of StreamingHandler into pipeline output.

[yuanwu2017]

Is your feature request related to a problem? Please describe.
We are using the haystack to setup a streaming chatbot demo with LLM. But haystack doesn't have the streaming mode RestAPI.
We found that the streaming mode RestAPI needs a streaming generator to get the tokens from handler.

Describe the solution you'd like

1. Add the streaming mode parameters in the promptNode run function, support the dynamically enabling and disabling.
2. Add the TextIteratorStreamer object into pipeline output and run the pipe() of prompt node in new thread. It can simplify the implementation of streaming RestAPI and support the non-blocked mode.

Describe alternatives you've considered
Implement a stremer handler(customer Iterator object) in rest_api and pass it in pipeline.run. It needs to run entire pipeline in other thread or coroutine.

Additional context
Add any other context or screenshots about the feature request here.

[vblagoje]

@yuanwu2017 @julian-risch I suggest we don't close this feature request but add it to the REST API task for V2 as a reminder that users ask for it

[julian-risch]

@vblagoje sounds fair 👍 Will be good to have this use case in mind once we re-design the REST API.

[masci]

Blocked by deepset-ai/hayhooks#9

[masci]

Won't fix in 1.x, let's use deepset-ai/hayhooks#9 to track the feature in 2.x