Support Cloudfront-Forwarded-Proto as alias for X-Forwarded-Proto

Amazon Cloudfront obnoxiously doesn't support passing X-Forwarded-Proto see http://docs.aws.amazon.com/AmazonCloudFront/latest/DeveloperGuide/header-caching.html#header-caching-web-protocol

in Nexus 2 you could configure the url manually, it wasn't advised but it allowed you to go https only with cloudfront for termination/caching. With Nexus 3 https://books.sonatype.com/nexus-book/reference/install-sect-proxy.html this doesn't seem to be an option, and neither does customizing the header.

In order for us to upgrade to Nexus 3 we'd like to run it behind cloudfront without a 2nd reverse proxy whose only purpose is to translate this header.

I don't know if a 2nd report would be necessary, but we'd need this in the docker image as well.