Reworked encryption to use less heap allocated buffers for secrets.

Also some work on plugin system.

IdentityShroud.TestUtils/Substitutes/NullDekEncryptionService.cs

@@ -11,6 +11,16 @@ public class NullDekEncryptionService : IDekEncryptionService
         return new(KeyId, plain.ToArray());
     }
 
+    public void Decrypt(EncryptedDek input, Span<byte> output)
+    {
+        input.Value.CopyTo(output);   
+    }
+
+    public int GetDecryptedSize(EncryptedDek input)
+    {
+        return input.Value.Length;
+    }
+
     public byte[] Decrypt(EncryptedDek input)
     {
         return input.Value;