Updated some interfaces, made time syncing methods nullable for apis not using it, added optional retry checking, removed private key from api credentials, added better support for api credentials subclasses

CryptoExchange.Net/Interfaces/IRestApiClient.cs

@@ -0,0 +1,33 @@
+using CryptoExchange.Net.Objects;
+using System;
+
+namespace CryptoExchange.Net.Interfaces
+{
+    /// <summary>
+    /// Base rest API client
+    /// </summary>
+    public interface IRestApiClient : IBaseApiClient
+    {
+        /// <summary>
+        /// The factory for creating requests. Used for unit testing
+        /// </summary>
+        IRequestFactory RequestFactory { get; set; }
+
+        /// <summary>
+        /// Total amount of requests made with this API client
+        /// </summary>
+        int TotalRequestsMade { get; set; }
+
+        /// <summary>
+        /// Get time offset for an API client. Return null if time syncing shouldnt/cant be done
+        /// </summary>
+        /// <returns></returns>
+        TimeSpan? GetTimeOffset();
+
+        /// <summary>
+        /// Get time sync info for an API client. Return null if time syncing shouldnt/cant be done
+        /// </summary>
+        /// <returns></returns>
+        TimeSyncInfo? GetTimeSyncInfo();
+    }
+}