Allow primative values in responses only

[pipermerriam]

from #89

What was wrong?

It is cumbersome to define a response event for things that have a primative singular value.

How can it be fixed?

We can allow the following primative types to be sent across the wire but only as a response to a request.

• int
• float
• bytes
• str

We can still package them up in an internal event like this:

class _PrimativeValue(BaseEvent):
    value: Union[int, float, bytes, str]

On the sending side, we only allow sending if the event is accompanied by a BroadcastConfig that defines filter_event_id and filter_endpoint.

On the receiving side, we unwrap the value if these conditions are met. This looks like this will require unwrapping in _process_item which means our various queues and things will have to have flexible type hints for the mechanisms where we're sending things to subscription handlers, futures, etc.