header enrichment policy

at the moment the only information that can be dynamically included with the header policy is the one retrieved or cached by the gateway.
Sometimes it is required to retrieve the information from another system and add it as header (or even adding 3scale specific information like Account or Application plan details, Mapping Rule matched, Service Id etc).

The scenario in this case would be:
policy applicable only before applying apicast policy
apicast send a rest request to an external system, asking for a specific info
apicast received the information on the requested Header
apicast allows you to redefine the Headers' value before forwarding these to the backend