feat: new tuple constructor with preallocated buffer

Summary

The constructor for small-allocated tuples, box_tuple_new(), implies copying the msgpack buffer into the tuple's memory. However, we'd like to avoid this intermediate buffer allocation and instead create a small-allocated tuple with a preallocated buffer of the required size, into which the necessary msgpack data can be written directly. To achieve this, the function small_tuple_prepare_buffer_ext() was created and exported.

Docs follow-up: not necessary