Option to disable adding extra fields by input plugins

So some plugins on top of setting up "type" add their own metadata, like TCP plugin adding source port in `port` field.

On top of being.... utterly pointless information (because source port without source IP is basically useless when trying to debug anyting) it makes it impossible to spot if `port` field came from JSON inside of connection or from plugin itself. There should be an option to turn extra info off.