Implement inbound PPv2

https://www.haproxy.org/download/1.8/doc/proxy-protocol.txt

This is a proxy forwarder protocol primary used by layer 4 proxies to forward information about the original client such as their IP. It does so by pre-pending the connection stream with a text or binary formatted blob of data. It's more efficient than adding X-Forwarded-For and similar headers to every request.

In Kestrel this would be implemented as a connection middleware. We helped an internal team build this middleware already and they're using it with an existing azure network services that sends that data.

I'll file a separate issue for outbound support as it would need to integrate with HttpClient.