Minor redesign and optimization of (Private)Key classes

[drieseng]

• Remove _privateKey field from Key base class.
• Remove ctor(byte[] data) from Key, and move current implementation to RsaKey and DsaKey classes.
• The ctor(byte[] data of RsaKey and DsaKey can be optimized to remove intermediate List<BigInteger>.
• Remove initial assignment of _publicKey from ED25519Key(byte[] pk, byte[] sk)ctor.
• Rename privateKey field in ED25519Key to _privateKey and remove disable of IDE1006 for this field.

[drieseng]

cc: @WojciechNagorski

[WojciechNagorski]

I don't mind. It looks like a great idea. IDE1006 is also a good role.

[WojciechNagorski]

The 2023.0.1 version has been released to Nuget: https://www.nuget.org/packages/SSH.NET/2023.0.1