Type information incorrect when @cached methods are passed as parameters

This issue has been migrated from [#16606](https://github.com/matrix-org/synapse/issues/16606).

---

If a method that is wrapped in `@cached` is passed into another function, e.g. `run_in_background` then the type information is incorrect; the current mypy plugin only updates the type information when it is called as a method, not when it is accessed as an attribute.

E.g. from #16590:

```
synapse/push/bulk_push_rule_evaluator.py:359: error: Missing positional argument "room_id" in call to "run_in_background" [call-arg]
synapse/push/bulk_push_rule_evaluator.py:360: error: Argument 2 to "run_in_background" has incompatible type "str"; expected "RoomMemberWorkerStore" [arg-type]
```

The method is not getting treated as bound (and there might be other issues too).