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.Core.Tests/JwtSignatureGeneratorTests.cs

@@ -72,8 +72,8 @@ public class JwtSignatureGeneratorTests
         var rsa = RSA.Create();
         var parameters = new RSAParameters
         {
-            Modulus = WebEncoders.Base64UrlDecode(jwk.Modulus),
-            Exponent = WebEncoders.Base64UrlDecode(jwk.Exponent)
+            Modulus = WebEncoders.Base64UrlDecode(jwk.Modulus!),
+            Exponent = WebEncoders.Base64UrlDecode(jwk.Exponent!)
         };
         
         rsa.ImportParameters(parameters);