feat(argon2): support salt input on Argon2

[Mikescops]

Hello,

I tried to implement supporting a pre-defined salt for the argon2 hashing.
I'm not familiar with Rust so I struggle to manage the conversion from the Js Buffer and the Rust SaltString, there are some unrecognized chars in the buffer.

I'm not sure why but this package isn't using the types defined in the documentation so I tried a few things without success.

Let me know if it's doable, that will be really helpful for us to have the salt supported 👍

[Mikescops]

Update:

I made a second commit because I noticed the difference between hashRaw and hash.
Because hashRaw is not using the SaltString type it is possible to directly use the buffer from the JS converted to a vec.
So it works for the raw version but for the "normal" (string) one, i can't figure it out.

[Brooooooklyn]

Thanks!