Fix joining remote rooms when a on_new_event callback is registered

[sandhose]

Since Synapse 1.76.0, any module which registers a on_new_event callback would brick the ability to join remote rooms.
This is because this callback tried to get the full state of the room, which would end up in a deadlock.

Related: matrix-org/synapse-auto-accept-invite#18

The following module would brick the ability to join remote rooms:

from typing import Any, Dict, Literal, Union
import logging

from synapse.module_api import ModuleApi, EventBase

logger = logging.getLogger(__name__)

class MyModule:
    def __init__(self, config: None, api: ModuleApi):
        self._api = api
        self._config = config

        self._api.register_third_party_rules_callbacks(
            on_new_event=self.on_new_event,
        )

    async def on_new_event(self, event: EventBase, _state_map: Any) -> None:
        logger.info(f"Received new event: {event}")

    @staticmethod
    def parse_config(_config: Dict[str, Any]) -> None:
        return None

This is technically a breaking change, as we are now passing partial state on the on_new_event callback.
However, this callback was broken for federated rooms since 1.76.0, and local rooms have full state anyway, so it's unlikely that it would change anything.

[sandhose]

I removed this method as it was just calling another one with less parameters available

[MatMaul]

We may want to add a boolean to the callback to inform the module if the state is complete or not.

[sandhose]

It seems it would be annoying to pass down. How useful would that be anyway? If the state is partial, you shouldn't try to load the full state anyway, else you'd deadlock

[reivilibre]

We can't add a boolean argument to the callback in a backwards-compatible way either. I'd like it out of purity otherwise, but I also can't easily think of why it'd be important

[sandhose]

I'll keep it like that for now, if that's really useful we can add that later

[reivilibre]

LGTM