Improve the `data-transformer::cairo-types::CairoU256` creation logic

## Component the issue is related to
Data Transformer

## Issue
`CairoU256::from_bytes` accepts an unbound `u8` slice, panicking if it's shorter than 32 bytes and ignoring the extra bytes if it's longer.
The argument should be a fixed-sized slice - `&[u8; 32]`.
Usage in `cheatnet::runtime_extensions::forge_runtime_extension` should be adjusted to the new method.