Add the option to set custom HTTP headers in reverse-proxy responses.

This would require adding additional configuration options to set custom headers in both frontend and backend nodes.
A key usage of this feature is the ability to set the HSTS header when using VyOS reverse-proxy to terminate SSL.

Example of VyOS configuration:

set load-balancing reverse-proxy backend web-bk-01 http-response-headers Proxy-Backend-ID value web-bk-01
set load-balancing reverse-proxy service https-frontend http-response-headers Strict-Transport-security value 'max-age=31536000; includeSubDomains;'