added ability to set optional http client for preventing connection pool exhaustion https://stackoverflow.com/a/48778707/4993930

CryptoExchange.Net/Objects/Options.cs

@@ -1,6 +1,7 @@
 using System;
 using System.Collections.Generic;
 using System.IO;
+using System.Net.Http;
 using CryptoExchange.Net.Authentication;
 using CryptoExchange.Net.Interfaces;
 using CryptoExchange.Net.Logging;
@@ -126,7 +127,10 @@ namespace CryptoExchange.Net.Objects
         /// The time the server has to respond to a request before timing out
         /// </summary>
         public TimeSpan RequestTimeout { get; set; } = TimeSpan.FromSeconds(30);
-     
+        /// <summary>
+        /// http client
+        /// </summary>
+        public  HttpClient? HttpClient;
         /// <summary>
         /// ctor
         /// </summary>
@@ -134,7 +138,15 @@ namespace CryptoExchange.Net.Objects
         public RestClientOptions(string baseAddress): base(baseAddress)
         {
         }
-
+        /// <summary>
+        /// ctor
+        /// </summary>
+        /// <param name="baseAddress"></param>
+        /// <param name="httpClient">Shared http client</param>
+        public RestClientOptions(HttpClient httpClient, string baseAddress) : base(baseAddress)
+        {
+            HttpClient = httpClient;
+        }
         /// <summary>
         /// Create a copy of the options
         /// </summary>
@@ -150,7 +162,8 @@ namespace CryptoExchange.Net.Objects
                 LogWriters = LogWriters,
                 RateLimiters = RateLimiters,
                 RateLimitingBehaviour = RateLimitingBehaviour,
-                RequestTimeout = RequestTimeout
+                RequestTimeout = RequestTimeout,
+                HttpClient = HttpClient
             };
 
             if (ApiCredentials != null)