Transport abstraction

[Shatur]

I working on https://github.com/lifescapegame/bevy_replicon. It currently coupled with Netcode. Users started to ask me to decouple it (projectharmonia/bevy_replicon#61), but there are two problems:

1. I need to run my systems before sending and after receiving independent on transport.
2. I need conditions to be transport independent.

I think this could be solved on Renet side:

• For 1 provide labels for sending and receiving systems and inside transports add corresponding systems to these labels. It's not only simple, but also idiomatic for Bevy.
• For 2 store information about connectivity inside RenetClient. This will reduce duplicated conditions code inside transports.

[lucaspoffo]

2. Yeah, this is something that should be implemented. Now that we have multiple transport abstractions this is pretty clear. Adding a simple enum that returns Disconnected/Connected/Connecting should be enough, and the transport layer would need to update this whenever the client transport state changes.

3. This shouldn't be that hard to add