Split PrivateKeyFile into different implementations.

[scott-xu]

Abstract the implementations into IPrivateKeyParser.
It will be helpful when add new parsers, for example, PuTTY.
Prepare for #341, #431, #434, #510,

[Rob-Hague]

It will be helpful when add new parsers, for example, PuTTY.

Are you planning on adding that? I saw some argon2-related stuff added to BouncyCastle

cc @darinkes

[darinkes]

Would be neat for sure to have it as part of the main project.

I use https://github.com/kmaragon/Konscious.Security.Cryptography since I don't use the whole BouncyCastle Lib.
But should be easy to replace the few bits with BouncyCastle, I think.

[scott-xu]

I also noted argon2 is added into BouncyCastle.
I'll have a try to add PuTTY support when I have free time.