fix(proxy): forward X-Forwarded-Proto via insert_header; add e2e test

Root cause: upstream_request_filter was inserting x-forwarded-proto with
a raw headers.insert() call (via DerefMut) which only updates base.headers
but NOT the CaseMap. header_to_h1_wire zips CaseMap with base.headers, so
headers added without a CaseMap entry are silently dropped on the wire.

Fix: use insert_header() which keeps both maps in sync.

Also adds:
- src/lib.rs + [lib] section: exposes SunbeamProxy/RouteConfig/AcmeRoutes
  to integration tests without re-declaring modules in main.rs
- tests/e2e.rs: real end-to-end test — starts a SunbeamProxy over plain
  HTTP, routes it to a TCP echo backend, and asserts x-forwarded-proto: http
  is present in the upstream request headers
- Updated unit tests to verify header_to_h1_wire round-trip (not just that
  HeaderMap::insert works in isolation)

Signed-off-by: Sienna Meridian Satterwhite <sienna@sunbeam.pt>

src/main.rs

@@ -1,17 +1,16 @@
-mod acme;
 mod cert;
-mod config;
-mod proxy;
 mod telemetry;
 mod watcher;
 
+use sunbeam_proxy::{acme, config};
+use sunbeam_proxy::proxy::SunbeamProxy;
+
 use std::{collections::HashMap, sync::Arc};
 
 use anyhow::Result;
 use kube::Client;
 use pingora::server::{configuration::Opt, Server};
 use pingora_proxy::http_proxy_service;
-use proxy::SunbeamProxy;
 use std::sync::RwLock;
 
 fn main() -> Result<()> {