Hooks for Pubisher and Subscriber

## Brief feature description

With the user-header it is possible to add custom meta information alongside the user-payload, e.g. a timestamp. It might be helpful to add some hooks to the Publisher and Subscriber to make this more ergonomic.

## Detailed information

The Publisher could have an `allocateHook` where the user-header could be initialized and a `deliveryHook` where the data could be set, e.g. a timestamp. On the Subscriber side there could be a `receiveHook` which can be used to evaluate the user-header.

Additionally to the user-header, this might also be useful to do a serialization/deserialization for dynamic size payloads.

It's not quite sure if this fits into the general concept and needs further discussions before this is implemented.