feat: external events

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
```ts
const session = await storage.read(chatId)
await fireEvent('id', conversation, session)
```
which loads the right session data, and runs the supplied conversation function.

Start by tracing how conversations currently wait for Telegram updates, then examine the proposed entry points `conversation.waitExternal('id')`, `storage.read(chatId)`, and `fireEvent('id', conversation, session)`. No files or tests are named, so first identify the conversation and storage paths involved. Done means an external event can resume the correct session and run the supplied conversation function.

Written by the indexing model from the issue text.