feat: external events

[KnorpelSenf]

It may be interesting to be able to wait for external events, rather than just waiting for Telegram updates.

For example, we could add something like await conversation.waitExternal('id') and then in turn people can do

const session = await storage.read(chatId)
await fireEvent('id', conversation, session)

which loads the right session data, and runs the supplied conversation function.