feat: rename "reverse proxy authentication" to "external authentication" (#4418)

* Rename external auth options

ReverseProxyWhitelist was regularly confusing users that enabled it for
non-authenticating reverse proxy setups.

The new option name makes it clear that it's related to authentication, not
just reverse proxies.

* small refactor

Signed-off-by: Deluan <deluan@navidrome.org>

* add test

Signed-off-by: Deluan <deluan@navidrome.org>

---------

Signed-off-by: Deluan <deluan@navidrome.org>
Co-authored-by: Deluan Quintão <deluan@navidrome.org>

server/subsonic/middlewares.go

@@ -56,7 +56,7 @@ func fromInternalOrProxyAuth(r *http.Request) (string, bool) {
 		return username, true
 	}
 
-	return server.UsernameFromReverseProxyHeader(r), false
+	return server.UsernameFromExtAuthHeader(r), false
 }
 
 func checkRequiredParameters(next http.Handler) http.Handler {