add please aes-xts256

[sidielnikov]

Samuel please make AES-XTS 256 on the entire Internet there is no normal source on C#
Here are a few examples

https://github.com/tkp1n/ArmAes
not working decryption. If it's not difficult to tell me how to fix the function
DecryptDataUnit256(
in AesKey key,
ref byte tweak,
ref byte ct, nuint ctLen,
ref byte pt)

https://github.com/mikhail-khalizev/XTSSharp

[samuel-lucas6]

This repo is for AEGIS, not AES modes in general. I don't currently have XTS on my private todo roadmap, and even if I did, it would be a long way off. I'd recommend looking for an implementation in a similar language like Go or Java. If the code is readable, it should be possible to port over.