Capture all HTTP Headers and make them available for custom notifications

[mapedraza]

Is your feature request related to a problem / use case? Please describe. After merging https://github.com/telefonicaid/fiware-orion/pull/4166, that make available FIWARE-Service, FIWARE-Service-Path and x-auth-token for custom notifications, the idea of having this functionality for all headers comes into play.

Describe the solution you'd like Using a modifier with a static part + the name of the header. I.E: ${_HEADER_my_header} where the returned value will be my_header content

Describe alternatives you've considered The static part of the token used can be different (_HEADER, or just _)

Describe why you need this feature

• To approach a new use case. Using different headers and capturing information from other GEs passed as headers

Additional information

[fgalan]

Apart from fiware-service, firware-servicepath and x-auth-token (which would be coverd by issue https://github.com/telefonicaid/fiware-orion/issues/4159 and associated PR https://github.com/telefonicaid/fiware-orion/pull/4166) which other headers could be interesting to capture from a NGSIv2 update?

Let's take into account that the only three meaningful headers from a API point of view are these ones. The update could be "decorated" with other headers (coming from intermediate proxies, etc.) but not sure which particular headers of such kind could be useful to manipulate in a custom notification....