[Server] Parse the rightmost address from forwarded header lists (#7586)

X-Forwarded-For carries a comma-separated list, and constructing a
QHostAddress from the raw header value returns a null address for it, so a
configured X-Forwarded-For or a multi-hop header never resolved and websocket
clients stayed on the proxy's address.

- Add parseForwardedAddress, returning the rightmost usable entry of a
  forwarded header list
- Use it for the websocket handshake instead of parsing the raw value
- Exempt loopback from per-address auth rate limits so a same-host proxy with a
  customized trusted_sources list cannot lock every user out
- Add forwarded_address_test

Co-authored-by: Lukas Brübach <Bruebach.Lukas@bdosecurity.de>

servatrice/CMakeLists.txt

@@ -7,6 +7,7 @@ project(Servatrice VERSION "${PROJECT_VERSION_MAJOR}.${PROJECT_VERSION_MINOR}.${
 set(servatrice_SOURCES
     src/email_parser.cpp
     src/event_loop_watchdog.cpp
+    src/forwarded_address.cpp
     src/main.cpp
     src/metrics_registry.cpp
     src/ratelimiter.cpp