batch send: use classes without unnecessary/unknown fields

[sumnerevans]

The batch send endpoint now accepts events and state events that don't
have room_id, event_id, etc. All batch send classes allow for the
specification of the timestamp.

[tulir]

Seems unnecessarily duplicating, and event type specific classes are only useful for deserialization. Something like this should be enough classes:

@dataclass
class BatchSendEvent(BaseEvent, SerializableAttrs):
    sender: UserID
    timestamp: int = attr.ib(metadata={"json": "origin_server_ts"})
    content: Any

@dataclass
class BatchSendStateEvent(BatchSendEvent, SerializableAttrs):
    state_key: str