CancelAllOrdersAsync can support multiple pair

[yaroslavlsd]

8d26e25
https://www.htx.com/en-us/opend/newApiPages/?id=8cb89359-77b5-11ed-9966-193afaf3b53

#region Cancel All Orders
+
+        /// <inheritdoc />
+        public async Task<WebCallResult<HTXCancelAllResult>> CancelAllOrdersAsync(IEnumerable<string>? symbols = null, CancellationToken ct = default)
+        {
+            //todo test me
+
+            var parameters = new ParameterCollection();
+            parameters.AddOptionalParameter("symbol", symbols == null ? null : string.Join(",", symbols.Select(x => x.ToLowerInvariant())));
+
+            var request = _definitions.GetOrCreate(HttpMethod.Post, "v1/order/cancel-all-orders", HTXExchange.RateLimiter.EndpointLimit, 1, true,
+                new SingleLimitGuard(1, TimeSpan.FromSeconds(2), RateLimitWindowType.Sliding, keySelector: SingleLimitGuard.PerApiKey));
+            return await _baseClient.SendBasicAsync<HTXCancelAllResult>(request, parameters, ct).ConfigureAwait(false);
+        }
+
+        #endregion