Allow specifying the ReceiveMessageBuffer size on Websockets (#228)

In order to support more User websocket connections, allow reducing the memory requirements for the receive buffer, keeping the default buffer.

CryptoExchange.Net/Objects/Sockets/WebSocketParameters.cs

@@ -64,6 +64,11 @@ namespace CryptoExchange.Net.Objects.Sockets
         /// </summary>
         public Encoding Encoding { get; set; } = Encoding.UTF8;
 
+        /// <summary>
+        /// The buffer size to use for receiving data
+        /// </summary>
+        public int? ReceiveBufferSize { get; set; } = null;
+
         /// <summary>
         /// ctor
         /// </summary>