Change the callback into a trait

[herr-seppia]

Describe what you want implemented
The on_message parameter should be a trait instead of fn(Vec<u8>)

Describe "Why" this is needed
Binding the callback to a simple function is useless in most scenarios. A trait is simply more manageable due to the self context availability

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

Additional context
Add any other context or screenshots about the feature request here.