AssetParsing improvements

[dartasen]

The original idea was to understand why the server loading could seem infinite when I try to start a server, I had assumed the possibility of race condition since we're using parallelism at some point but in this current state it was quite complicated to determine given the current implementation which has a rather confusing control flow. So I took the opportunity to try to clean this up while trying both investigate issues & upgrade to the latest version of AssetTools

Progress :

• [x] Bump to latest version of AssetTools
• [x] Refactor asset parsers to avoid using static constructors in favor to Disposable parsers, since Nitrox is using them for a one shot load
• [x] Drop System.Drawing reference in favor of ImageSharp which is cross-platform for RandomSpawnTexture reading
• [ ] Improve UX by adding more informations about what's going behind the scene (especially when game is on a slow external disk, it can takes several minutes to load)
• [ ] Fix loading errors happening with the refactor