question: dynamic upstream and routing for proxy

[georglauterbach]

What is the problem your feature solves, or the need it fulfills?

Given this code:

fn main() {
    let mut server =
        pingora::server::Server::new(Some(pingora::server::configuration::Opt::parse_args()))
            .unwrap();
    server.bootstrap();

    let mut proxy = pingora::proxy::http_proxy_service(&server.configuration, Gateway);
    proxy.add_tcp("0.0.0.0:3456");
    server.add_service(proxy);

    server.run_forever();
}

#[derive(Debug)]
pub struct Gateway;

#[async_trait::async_trait]
impl pingora::proxy::ProxyHttp for Gateway {
    type CTX = ();
    fn new_ctx(&self) -> Self::CTX {}

    async fn upstream_peer(
        &self,
        _session: &mut pingora::proxy::Session,
        _ctx: &mut Self::CTX,
    ) -> pingora::Result<Box<pingora::upstreams::peer::HttpPeer>> {
        let peer = Box::new(pingora::upstreams::peer::HttpPeer::new(
            ("127.0.0.1", 80),
            false,
            "".to_string(),
        ));
        Ok(peer)
    }
}

I'd simply like to know how I would best go about adding new upstreams and configuration dynamically (during run-time). I read in #250 (comment) that it should be possible already.

CC @eaufavor