Add PaginatedQuery abstract class, change startIndex to skip

Jellyfin.Data/Queries/PaginatedQuery.cs

@@ -0,0 +1,18 @@
+namespace Jellyfin.Data.Queries
+{
+    /// <summary>
+    /// An abstract class for paginated queries.
+    /// </summary>
+    public abstract class PaginatedQuery
+    {
+        /// <summary>
+        /// Gets or sets the index to start at.
+        /// </summary>
+        public int? Skip { get; set; }
+
+        /// <summary>
+        /// Gets or sets the maximum number of items to include.
+        /// </summary>
+        public int? Limit { get; set; }
+    }
+}