Add optional src and dst address for `TransportPacket*` events

[lnicco]

Add optional src and dst address for TransportPacketSent, TransportPacketReceived, TransportPacketDropped, TransportPacketBuffered

Fixes #57

[lnicco]

Actually this should be for the Datagram events and not for Packet events TransportDatagramsSent TransportDatagramsReceived TransportDatagramDropped

[rmarx]

As mentioned in #57, I would rather see this (and some of the other events we have that list IPs/ports, like ConnectivityConnectionStarted) updated to a generic PathInfo struct (or similar).

This does depend a bit on how we approach migration events (as they probably want to re-use this too). (also depends on multipath approach, but since it looks like we're not doing that, we can skip it).

In short: this is not yet ready for review/merge atm :)

[LPardue]

Overtaken by #336?

[rmarx]

This was indeed solved (or at least changed in a different way) by #336, so closing without merging.