Add advanced configurations for Access logs

At the moment enabling and disabling are the only options for Access log. But If we are to use an LB like AWS ALB, cannot depend on the access log of the registry because it'll log the IP address of the ALB as the Host instead of the client. Although ALB sends the client IP with "X-Forwarded-For" HTTP Header. if we can have a configuration to extract custom header to log, it would be great !.