Python: supporting ASGI legacy protocol.

Introducing manual protocol selection for 'universal' apps and frameworks.

src/nxt_application.h

@@ -51,6 +51,7 @@ typedef struct {
     nxt_str_t  path;
     nxt_str_t  module;
     char       *callable;
+    nxt_str_t  protocol;
     uint32_t   threads;
     uint32_t   thread_stack_size;
 } nxt_python_app_conf_t;