Feature: Only intercept/manipulate packets of interest

[dorving]

Add a way for extensions to specify the packets it is interested in. That way the extension server doesn't have to forward every single packet, but only those the extension actually is interested in.

I would like to work on this once my PR for rewriting the network extensions code (#137) is merged.

[sirjonasxx]

since this would probably break backward compatibility with older extensions, we need to make sure this has practical benefit, perhaps with some small benchmarking

[dorving]

I was thinking we we could add an optional NetworkMessage with a list of all the packet identifiers the extensions needs.
Then the network server only forwards those, in case it receives no such message from the client, it just operates as it did before. So backward compatibility should remain intact. I think