Cleanup roots of unity in KZGSettings

[asn-d6]

This PR makes some steps towards https://github.com/ethereum/c-kzg-4844/issues/439

Leftover things to do:

• Completely remove max_width. This is not a variable field so it doesn't need to be in KZGSettings. It's a parameter of the system, and by having it as a variable it makes things more confusing. IMO we should just use FIELD_ELEMENTS_PER_EXT_BLOB directly where it's appropriate, and FIELD_ELEMENTS_PER_BLOB in FK20.
• At some point we should stop this domain_size + 1 artifact stemming from expand_root_of_unity() since it's needless and confusing. I decided to not do it because it would also change the 4844 code and I would ideally like it to remain as it was.

[asn-d6]

Thanks a lot for the help @jtraglia !