add json property purpose (#168)

Signed-off-by: Johannes Tuerk <johannes.tuerk@lissi.id>

Author: JoTiTu

src/WalletFramework.Oid4Vc/Oid4Vp/PresentationExchange/Models/PresentationDefinition.cs

@@ -36,6 +36,7 @@ public class PresentationDefinition
     /// <summary>
     ///     This MUST be a string that describes the purpose for which the Presentation Definition's inputs are being used for.
     /// </summary>
+    [JsonProperty("purpose")]
     public string? Purpose { get; }
 
     /// <summary>