Add steps to merge original request and enricher response headers

[deadlycoconuts]

This PR introduces a minor change to merge the headers of the original request sent to Turing routers with that of the response returned from an enricher.

This in effect parses the enricher response headers for downstream traffic rules and experiment input, but does not parse them for ensembling.

Modifications

• engines/router/missionctl/handlers/http_handler.go - Addition of a few lines of code to merge the headers of the original request and enricher response

[krithika369]

The change looks good. I think it would be useful to also log the enricher's response header in the outcome logs. This will require a change to the proto. We can tackle this as a separate item.