FIWARE-Correlator

[kzangeli]

Add the FIWARE-Correlator header to notifications, if present in the original request.This is not an NGSI.-LD thing,
it dates back to NGSIv2, but, that's OK.
Some day we'll discuss this feature in ETSI ISG CIM and we just might add something similar to NGSI-LD.

[kzangeli]

Workaround implemented in PR #1361
Kind of perfect workaround, but, I keep this issue to remind me to take the correlator concept to ETSI.