Support for upstream uwsgi protocol

It'd be nice to be able to support the uwsgi binary protocol for an upstream cluster when speaking to a service that uses this.

Fundamentally this is a lighter weight protocol when you're running a service behind uwsgi already, you can skip another layer of http processing and speak the native uwsgi protocol.

Prior art: nginx supports a `uwsgi_pass` directive: https://nginx.org/en/docs/http/ngx_http_uwsgi_module.html
Ref: https://uwsgi-docs.readthedocs.io/en/latest/Protocol.html