Return user header ptr instead of reference

## Brief feature description

The user header pointer should be returned in `Sample`, `Request`, `Response`.

To cite @elBoberido from #1081
I know you want to prevent to use a raw pointer her, but in this case it is error prone to use a reference. Consider the following scenario
```cpp
auto request = whatEverReturnsARequest();
auto reqHeader = request.getResponseHeader();
reqHeader.setFireAndForget();
request.send();
```
Do you spot the error?

We either have to use `std::reference_wrapper` or a similar implementation, maybe even with tracking of all the handed over references.
Currently I would favor to return the raw pointer

### Related to
[#1071 Replace raw pointer with std::reference_wrapper where applicable](https://github.com/eclipse-iceoryx/iceoryx/issues/1071)

Start by locating the C++ definitions and accessors for Sample, Request, and Response, then read the discussion in related issues #1081 and #1071 about references and raw pointers. Confirm how header lifetimes are handled before changing the return types; the work is done when all three APIs consistently return the intended user-header pointer and their existing usage remains valid.

Written by the indexing model from the issue text.