HTTP: added TSTR validation flag to the rewrite option.

This is to improve error messages for rewrite configuration.
Take the configuration as an example:

  {
      "rewrite": "`${a + "
  }

Previously, when applying it the user would see this error message:

  failed to apply previous configuration

After this change, the user will see this improved error message:

  the previous configuration is invalid: "SyntaxError: Unexpected end of input in default:1" in the "rewrite" value.

src/nxt_conf_validation.c

@@ -690,6 +690,7 @@ static nxt_conf_vldt_object_t  nxt_conf_vldt_action_common_members[] = {
     {
         .name       = nxt_string("rewrite"),
         .type       = NXT_CONF_VLDT_STRING,
+        .flags      = NXT_CONF_VLDT_TSTR,
     },
     {
         .name       = nxt_string("response_headers"),