UseForwardedHeaders should not manipulate headers

RE: https://github.com/microsoft/reverse-proxy/issues/642

## Summary

UseFowarardedHeaders makes some design choices that make it interop poorly with proxies like YARP. It's also inconsistent compared to how other middleware interact with the request.

## Motivation and goals

- The middleware modifies the incoming x-forwarded-* headers, making them difficult to append to and forward in a proxy.
- The middleware stores the original values in x-original-* headers, rather than putting them in a Feature like other components. These x-original-* headers end up getting proxied and the destination doesn't know what to do with them.

# Detailed design

- Still update HttpContext.Connection.RemoteIp/Port, Request.Scheme/Host, etc.
- Store the original values in a new feature
- Don't modify the request headers
- Don't add the x-original request header, store this data in a feature instead.

When proxy code wants to append to x-forwarded-* headers it would need to check for the new feature for the original values of scheme, host, port, etc.