Configurable Salt for AES for Deterministic Zips

[Philip-Trettner]

I'm building a reproducible/deterministic build/action system and some parts require password protected zip files.

Some builds were not reproducible and I could narrow it down to the password protected zips.

In particular, the AesWriter::new creates a salt using rand::thread_rng() (https://github.com/zip-rs/zip2/blob/master/src/aes.rs#L240).

As far as I understand it, this makes it impossible to have deterministic/reproducible zips with Aes as you cannot seed the thread_rng.

So my feature request for one of the following:

• add an option to provide a custom Rng
• add an option to provide a custom Seed
• add an option to provide a custom Salt
• add an option to fall back to a fixed Salt
• or some other option that makes the process deterministic

(I basically need only a single way to get determinism here. The password itself is derived from a build hash, so there is no security implication in my case afaict.)

I'd be happy to make a PR if you're ok with such an option and provide me with a preferred way.