add BEAMMP_PROVIDER_PORT_ENV to allow provider to change which ENV

variable the port is fetched from

include/Env.h

@@ -26,6 +26,7 @@ enum class Key {
     // provider settings
     PROVIDER_UPDATE_MESSAGE,
     PROVIDER_DISABLE_CONFIG,
+    PROVIDER_PORT_ENV,
 };
 
 std::optional<std::string> Get(Key key);