add singelton

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

Author: JoTiTu

src/WalletFramework.SdJwtVc/ServiceCollectionExtensions.cs

@@ -11,6 +11,7 @@ public static IServiceCollection AddSdJwtVcServices(this IServiceCollection buil
     {
         builder.AddSingleton<IHolder, Holder>();
         builder.AddSingleton<ISdJwtVcHolderService, SdJwtVcHolderService>();
+        builder.AddSingleton<ISdJwtSignerService, SdJwtSignerService>();
         return builder;
     }