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>

Cargo.toml

@@ -3,6 +3,10 @@ name    = "sunbeam-proxy"
 version = "0.1.0"
 edition = "2021"
 
+[lib]
+name = "sunbeam_proxy"
+path = "src/lib.rs"
+
 [dependencies]
 # Pingora with rustls backend (pure Rust TLS, no BoringSSL C build)
 pingora       = { version = "0.7", features = ["rustls"] }