[internal] Separate holding of GIL in create_digest_to_digest()

[Eric-Arellano]

Prework for #13526.

Before, we were parsing the Python objects (which requires the GIL) in the same iterator that created async blocks. That won't work when we port to PyO3 - iterating over the file_items collection uses &'py PyAny values, where the 'py lifetime corresponds to how long we are holding the GIL (as represented by the pyo3::Python type). The Python token is not safe in async blocks because it does not implement Send, so we can't create the futures in this iterator.

This fix makes the code much more understandable as well. In the future, we are unblocked from defining CreateDigest in Rust with PyO3 because parsing of the code is now completely decoupled from using it to create async futures.

[ci skip-build-wheels]