feat(pkg-py): Add new message_content() and message_content_chunk()

[cpsievert]

Adds new message_content() and message_content_chunk() generic functions (via singledispatch). They aren't intended to be called directly by users, but instead, provide an opportunity to teach Chat.append_message()/Chat.append_message_stream() how to extract a ChatMessage() from different bespoke objects.

from shiny import reactive
import shiny.express

from shinychat import message_content
from shinychat.express import Chat
from shinychat.types import ChatMessage

class MyObject:
    foo = "bar"

@message_content.register
def _(message: MyObject):
    return ChatMessage(content=message.foo, role="assistant")

chat = Chat("chat")
chat.ui()

@reactive.effect
async def _():
    await chat.append_message(MyObject())

These new functions are the singledispatch equivalent of the previous (internal) normalize_message() and normalize_message_chunk() helpers. Although not quite as flexible as the previous strategies pattern, this approach much simpler (implementation wise and also for those registering new methods) and aligns much better with shinychat::contents_shinychat() (#52).

[gadenbuie]

Looks great, just one small name adjustment comment. Thanks!