Reworked encryption to use less heap allocated buffers for secrets.

Also some work on plugin system.

IdentityShroud.Core.Tests/Services/EncryptionTests.cs

@@ -19,7 +19,8 @@ public class EncryptionTests
         byte[] keyValue = Convert.FromBase64String("IGd9yUMusjNW0ezv8ink3QWlAHKFH45d21LyrbJTokw=");
         
         // act
-        byte[] result = Encryption.Decrypt(cipher, keyValue);
+        byte[] result = new byte[Encryption.GetDecryptedLength(cipher)];
+        Encryption.Decrypt(cipher, keyValue, result);
 
         // verify
         Assert.Equal("Hello, World!"u8, result);