Replace raw pointer with `std::reference_wrapper` where applicable

## Brief feature description

Currently we use raw pointer at several locations in our code base. This requires us to do `nullptr` checks even when we know that it is not possible to get a `nullptr` when the API is used in the correct way. Just returning a reference and expecting a reference as argument is not always possible or might be the source for different errors. By using `std::reference_wrapper` or an own implementation if the STL type does not give us the desired behavior, we can communicate that we do not expect `nullptr` and make our code base more robust towards a wrong usage.

## Detailed information

A problem with returning plain references is a potential unintentional copy on the caller side, e.g.
```cpp
Foo& getFoo() {
return fooFromShm();
}

void releaseFoo(Foo& foo) {
// code
}

int main() {
auto foo = getFoo(); // copy occurs here
releaseFoo(foo); // release a stack based object
}
```
To prevent this, a `std::reference_wrapper` might be returned instead of a plain reference.

Places where this could be used are the `SubscriberPort`, `PublisherPort`, `ClientPort`, `ServerPort`, and probably many more places.

### To Do
- [ ] remove `cxx::not_null`

### Related to
[#717 Replace cxx::not_null with reference wherever possible](https://github.com/eclipse-iceoryx/iceoryx/issues/717)
[#1101 Return user header ptr instead of reference](https://github.com/eclipse-iceoryx/iceoryx/issues/1101)