Memory leak executing UnsignedTransaction.to_json()

Trying to sign transactions in a loop, the UnsignedTransaction to_json method does not free the memory used.

Attaching a test case ran with nodejs.

[mem-leak.txt](https://github.com/ergoplatform/sigma-rust/files/9747017/mem-leak.txt)

Start by reproducing the attached mem-leak.txt case in Node.js, which repeatedly signs transactions and calls UnsignedTransaction.to_json(). Then locate the implementation and its binding path, and compare memory usage across iterations. Done means the provided loop no longer shows the reported memory growth.

Written by the indexing model from the issue text.