Include x-correlator header

[fernandopradocabrillo]

Problem description
We need to include the x-correlator header as agreed in commonalities.

Design Guidelines: https://github.com/camaraproject/Commonalities/blob/main/documentation/API-design-guidelines.md#9-architecture-headers

[fernandopradocabrillo]

Hi @akoshunyadi
regarding the question that you raised this morning in the meeting about if the value of the x-correlator in the notification call should be the same one as in the subscription creation, we believe that it doesn't need to.
The correlator is used to track all the procceses and executions regarding one operation. But in this case we have two different operations, once being the creation of the subscription and the other one the post of the event.
The relation between the subscription and the event is the subscriptionId