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/Contracts/ISecretProvider.cs

@@ -1,6 +1,14 @@
+using IdentityShroud.Core.Security;
+
 namespace IdentityShroud.Core.Contracts;
 
 public interface ISecretProvider
 {
     string GetSecret(string name);
+
+    /// <summary>
+    /// Should return one active key, might return inactive keys.
+    /// </summary>
+    /// <returns></returns>
+    KeyEncryptionKey[] GetKeys(string name);
 }