feat: add ECC P-521 algorithm to PivAlgorithm

Author: DennisDyallo

Yubico.YubiKey/src/Yubico/YubiKey/Piv/PivAlgorithm.cs

@@ -85,11 +85,15 @@ public enum PivAlgorithm
         /// </summary>
         EccP384 = 0x14,
 
+        /// <summary>
+        /// Indicates that the algorithm is ECC and the parameters are P-521,
+        /// </summary>
+        EccP521 = 0x15,
+
         /// <summary>
         /// Indicates that the slot contains a PIN or PUK (slots 80 and 81).
         /// While not a cryptographic algorithm, it is used in the PIV Metadata.
         /// </summary>
         Pin = 0xFF,
-        EccP521 = 0x100
     }
 }