box: prepare to encode client error args as payload

With new harness we can specify in 'errcode.h' for example

```c
	_(ER_TUPLE_NOT_FOUND, 4, "Tuple doesn't exist", "space", STRING, "index", STRING) \
```

and then on

```c
	diag_set(ClientError, ER_TUPLE_NOT_FOUND, "space A", "index B")
```

the error set will have payload fields "space" and "index" with
corresponding values.

Closes #9109

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