Add metadata to on_message callback

[t00ts]

Added metadata to on_message callback as per #59

Added a MessageInfo struct within encoding module because imho it made sense for it to be there with other Message, Header, Payload, and other related types.

Using a tuple for actual message passing through the notification channel.

[herr-seppia]

LGTM

[t00ts]

For future reference:

Added a MessageInfo struct within encoding module because imho it made sense for it to be there with other Message, Header, Payload, and other related types.

We changed this and MessageInfo is now part of the handling module, as it's where messages are processed.