Add project files.

Core.Cerberos.Adapters/UserExistenceAdapter.cs

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+// ***********************************************************************
+// <copyright file="UserExistenceAdapter.cs">
+//     Heath
+// </copyright>
+// ***********************************************************************
+
+using System.Text.Json.Serialization;
+
+namespace Core.Cerberos.Adapters
+{
+    /// <summary>
+    /// Adapter representing a user.
+    /// </summary>
+    public class UserExistenceAdapter
+    {
+        /// <summary>
+        /// user existence.
+        /// </summary>
+        [JsonPropertyName("existence")]
+        public bool Existence { get; set; }
+    }
+}