Support Upcasting of Headers (or Messages)

[fritz-gerneth]

Right now, upcasting is limited to the event data itself. We have a case now, where we would like to upcast headers (add tenant information). As we are using our own serializers, we added the support for this in our own headers serializer.

Wondering though if it makes sense to add this as a general feature - or just pass the entire Message to upcasters instead? The later would keep the implementation cleaner (no separation between different kinds of upcasters).

[DanielBadura]

Sounds reasonable to add some sort of upcasting also for headers. The data structure could also be mostly the same as the current upcaster.

I think for 4.0.x we could also consider passing the whole payload into an upcasting process, so that we can upcast whole messages or parts of the message. WDYT @DavidBadura ?

[DanielBadura]

I also created another issue which goes into a similiar direction, it's about removing unused headers: #818