Firestore functions doesn't ignore `/` prefix

In JS/Node SDK, both form of document path are accepted:

```
document="/messages/{messageId}"
document="messages/{messageId}"
```

This is done by normalizing document path ([code](https://github.com/firebase/firebase-functions/blob/47cfc61b52448e711af9261722049af58471b545/src/common/utilities/path.ts#L6-L11)).

This "magic" is missing in the python sdk.