[C#] SharedCryptoKeyEntry efficiency and modernization

Evaluate the Crypto.Keys.SharedCryptKeyEntry for type efficiency and modernization. I believe that the CachedTimeMillis is both a poor name and a suboptimal type, whereas the class could use better DateTime types to avoid all the usage of ToUnixTimeMilliseconds. Consider some better naming as well as CachedTimeMillis sounds like a duration but its a timestamp.

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