Allow appending objects to the ChatMessage header & footer

[ahuang11]

If user runs serialize the contents will be separate from the footer/header objects, and reduces the need to write a custom serializer.

import panel as pn
from openai import AsyncOpenAI

pn.extension()


async def callback(contents: str, user: str, instance: pn.chat.ChatInterface):
    response = await aclient.chat.completions.create(
        model="gpt-3.5-turbo",
        messages=[{"role": "user", "content": contents}],
        stream=True,
    )
    parts = ""
    indicator = pn.indicators.Dial(name="Length")
    async for chunk in response:
        part = chunk.choices[0].delta.content
        if part:
            parts += part
            indicator.value = len(parts)
            yield pn.chat.ChatMessage(
                parts,
                user="ChatGPT",
                header_objects=[
                    pn.Row(
                        pn.widgets.FileDownload(icon="download", margin=(0, 5, 5, 15)),
                    )
                ],
                footer_objects=[indicator],
            )

aclient = AsyncOpenAI()
chat_interface = pn.chat.ChatInterface(callback=callback, callback_user="ChatGPT")
chat_interface.send(
    "Send a message to get a reply from ChatGPT!", user="System", respond=False
)
chat_interface.servable()

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 71.66%. Comparing base (39379c8) to head (df2f2c0).
Report is 12 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #6410       +/-   ##
===========================================
- Coverage   83.63%   71.66%   -11.98%     
===========================================
  Files         305      305               
  Lines       45594    45739      +145     
===========================================
- Hits        38132    32777     -5355     
- Misses       7462    12962     +5500     

Flag	Coverage Δ
ui-tests	?
unitexamples-tests	71.66% <100.00%> (+0.04%)	⬆️

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