feat: add a REST API streaming endpoint for the query pipeline

[faaany]

Related Issues

Following the suggestion in PR #4889, I implemented a streaming endpoint in my application. I thought this feature might be interesting for other users as well, which leads to this PR.

Proposed Changes:

• add a new endpoint query-streaming, which outputs the result of the last component in the pipeline in streaming form
• this feature has one prerequisite: the last node in the pipeline should be a PromptNode or a node that accepts stream_handler in their run method. Otherwise, a warning message will be returned.

How did you test it?

• locally with the following example yaml file

components:
- name: AnswerParser
  params: {}
  type: AnswerParser
- name: custom-at-query-time
  params:
    output_parser: AnswerParser
    prompt: "Please answer the following question. Question: {query} \n\n Answer:"
  type: PromptTemplate
- name: generator
  params:
    api_key: <your openai api key>
    default_prompt_template: custom-at-query-time
    model_name_or_path: text-davinci-003
  type: PromptNode
pipelines:
- name: query
  nodes:
  - inputs:
    - Query
    name: generator
version: 1.20.0rc0

To start the backend, run

export PIPELINE_YAML_PATH=/home/user/workspace/example.yaml 
export QUERY_PIPELINE_NAME=query 
uvicorn rest_api.application:app --reload --workers 1

To query the results, run

curl -w "\n" -N -X POST -H "Content-Type: application/json" -d '{"query": "Why do airplanes leave contrails in the sky?"}' http://localhost:8000/query-streaming

Notes for the reviewer

There is another PR #5646 of mine which enables passing an object to pipeline.run(). This PR will only work if that PR gets merged first. @ZanSara

The "exception handling" in my solution (the warning message) is not optimal, pls let me know if you have a better idea.
Also please let me know if a unit test is needed here.

Checklist

• I have read the contributors guidelines and the code of conduct
• I have updated the related issue with new insights and changes
• I added unit tests and updated the docstrings
• I've used one of the conventional commit types for my PR title: fix:, feat:, build:, chore:, ci:, docs:, style:, refactor:, perf:, test:.
• I documented my code
• I ran pre-commit hooks and fixed any issue

[faaany]

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