5-improve-encrypted-storage (#6)

Added the use of DEK's for encryption of secrets. Both the KEK's and DEK's are stored in a way that you can have multiple key of which one is active. But the others are still available for decrypting. This allows for implementing key rotation.

Co-authored-by: eelke <eelke@eelkeklein.nl>
Co-authored-by: Eelke76 <31384324+Eelke76@users.noreply.github.com>
Reviewed-on: #6

IdentityShroud.TestUtils/Substitutes/NullDataEncryptionService.cs

@@ -0,0 +1,18 @@
+using IdentityShroud.Core.Contracts;
+using IdentityShroud.Core.Security;
+
+namespace IdentityShroud.TestUtils.Substitutes;
+
+public class NullDataEncryptionService : IDataEncryptionService
+{
+    public DekId KeyId { get; } = DekId.NewId(); 
+    public EncryptedValue Encrypt(ReadOnlySpan<byte> plain)
+    {
+        return new(KeyId, plain.ToArray());
+    }
+
+    public byte[] Decrypt(EncryptedValue input)
+    {
+        return input.Value;
+    }
+}