feat: re-add multi to target session

[driftluo]

ref #305

After a period of online testing and use, we found that we needed to add back Multi

The current methods of sending messages can be divided into two main categories:

1. sending with a specified id (Single, Multi)
2. send all (All, Filter)

For multicast where the specified object is known, using Filter is a costly act and a full traversal of all connected nodes must be performed.

Filter is more suitable for behaviors where the specific target is not known, but the sending rules are known, such as gossip

Now there is one more problem, do we need to specify Vec or the more loose Box<dyn Iterator<Item = SessionId> + Send + 'static> as the type of Multi？ @TheWaWaR @quake