feat: accept chunks as arguments to chat.{start,append,stop}Stream methods

[zimeg]

Summary

This PR introduces the chunks argument to the following methods:

• chat.startStream
• chat.appendStream
• chat.stopStream

⚠️ This feature is experimental at the moment!

Testing

The following code snippet might be interesting to experiment with:

import time
...

streamer = client.chat_startStream(
    channel=channel_id,
    recipient_team_id=team_id,
    recipient_user_id=user_id,
    thread_ts=thread_ts,
    chunks=[
        MarkdownTextChunk(text="**onwards processing**"),
        TaskUpdateChunk(
            id="12",
            title="counting bytes...",
            status="in_progress",
        ),
    ],
)

time.sleep(4)

client.chat_appendStream(
    channel=channel_id,
    ts=streamer.get("ts"),
    markdown_text="",
    chunks=[
        TaskUpdateChunk(
            id="12",
            title="adding numbers...",
            status="in_progress",
            details="sums have increased",
        )
    ],
)

time.sleep(4)

client.chat_stopStream(
    channel=channel_id,
    ts=streamer.get("ts"),
    chunks=[
        TaskUpdateChunk(
            id="12",
            title="solved equation!",
            status="complete",
            sources=[
                URLSource(
                    url="https://oeis.org",
                    text="The On-Line Encyclopedia of Integer Sequences (OEIS)",
                ),
            ],
        ),
        MarkdownTextChunk(text="that computes."),
    ],
)

Category

• [x] slack_sdk.web.WebClient (sync/async) (Web API client)
• [x] slack_sdk.models (UI component builders)
• [ ] /docs (Documents)
• [ ] tests/integration_tests (Automated tests for this library)

Notes

• Planning to add tests alongside these methods soon!
• Unsure if these are the best naming for "models" and I'm open to suggestions!

Requirements

• [x] I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• [x] I've read and agree to the Code of Conduct.
• [ ] I've run python3 -m venv .venv && source .venv/bin/activate && ./scripts/run_validation.sh after making the changes.