Expose `box_tuple_data` API for cases where introducing copies with
`box_tuple_to_buf` is undesirable.

Fixes tarantool#11478

@TarantoolBot document
Title: `box_tuple_data` to get a pointer to the underlying MsgPack data.

Added new C function `box_tuple_data` to inspect raw MsgPack data. It is
similar to the `box_tuple_to_buf` but the former gives one an access to
the data pointer directly, not to a copy in a buffer. The 'bsize'
argument will be set to the `box_tuple_bsize` result if it is not NULL.

```C
const char * box_tuple_data(box_tuple_t *tuple, size_t *bsize);
```