wip draft: HKeyExchange rewrite

[PaulusParssinen]

This is just a work-in-progress draft PR to track my slow progress to attempt making the HKeyExchange implementation more "correct". I'm really suspicious of the current implementation because it's does the PKCS 1.5 padding incorrectly and it might have been working by pure luck (lol?). This piece of the SDK should also have really good testing coverage in future 😅

Having complete amateurs writing a managed crypto implementation is terrible idea and so I added a disclaimer remark for that too. We would like to use the more "correct" RSA implementations provided by the base OS but my attempts to do that have not go well so far.

So far I've added implementation of the probabilistic Miller-robin primality test for the random prime generation and rewrited the PKCS#1 1.5 padding to use very handy RandomNumberGenerator.GetNonZeroBytes(Span<byte> data) (which might be obsoleted in near future, see https://github.com/dotnet/runtime/issues/42763)

TODO

• Tests!
• Attempt to use the unmanaged RSA implementations over interop, which should(?) allow us to store the RSA key parameters as byte arrays and allow us to clear them on dispose. BigIntegers just will be left to GC to deal with.. which is bad and which is one of the many reasons this shit shouldn't exist in managed code.
• Make consuming the API from less painful from the game network protocol (to handle the difference between Java BigInteger)
• Investigate if it is possible to make this more abstract. I'd like to see if we can allow user to plug a different crypto algos and support other game client/server versions. I'm pretty interested how far even into shockwave version we can make it, if we can at all.
• Document the new API usage

[ArachisH]

Had to move the base to another branch for some spring(summer) cleaning, also what's up with that conflict?

[PaulusParssinen]

Not entirely sure what was up with the HKeyExchange conflict.. Will try finish this one ASAP.

[PaulusParssinen]

ggs, will continue on my fork.