Support rotation of master key.

The EncryptionService now loads a set of keys and uses the active one to encrypt and selects key based on keyid during decryption. Introduced EncryptedValue to hold keyId and encrypted data.

(There are no intermeddiate keys yet)

IdentityShroud.Core/Contracts/ISecretProvider.cs

@@ -3,4 +3,10 @@ namespace IdentityShroud.Core.Contracts;
 public interface ISecretProvider
 {
     string GetSecret(string name);
+
+    /// <summary>
+    /// Should return one active key, might return inactive keys.
+    /// </summary>
+    /// <returns></returns>
+    EncryptionKey[] GetKeys(string name);
 }