Add MedMind recovery and Steam authentication

Models/ExternalIdentityLink.cs

@@ -0,0 +1,16 @@
+using MongoDB.Bson;
+using MongoDB.Bson.Serialization.Attributes;
+using System;
+
+namespace GameAuthApi.Models;
+
+public class ExternalIdentityLink
+{
+    [BsonId]
+    [BsonRepresentation(BsonType.ObjectId)]
+    public string Id { get; set; }
+    public string Provider { get; set; }
+    public string Subject { get; set; }
+    public string UserId { get; set; }
+    public DateTime CreatedUtc { get; set; }
+}