Feature: support use Message object as message template

[mnixry]

demo code (with mirai adapter):

from nonebot.adapters.mirai import MessageEvent, Bot, MessageChain, MessageSegment
from nonebot.plugin import on_message

handler = on_message()


@handler.handle()
async def _(bot: Bot, event: MessageEvent, state: dict):
    await handler.send(MessageChain.template(
        MessageSegment.plain('test {event.source}') + MessageSegment.face(233) +
        MessageSegment.plain('test {event.message_chain}')).format(event=event),
                       at_sender=True)

[github-actions]

🚀 Deployed on https://deploy-preview-545--nonebot2.netlify.app