Added support for caching GET requests

CryptoExchange.Net/Caching/MemoryCache.cs

@@ -0,0 +1,54 @@
+using System;
+using System.Collections.Concurrent;
+
+namespace CryptoExchange.Net.Caching
+{
+    internal class MemoryCache
+    {
+        private readonly ConcurrentDictionary<string, CacheItem> _cache = new ConcurrentDictionary<string, CacheItem>();
+
+        /// <summary>
+        /// Add a new cache entry. Will override an existing entry if it already exists
+        /// </summary>
+        /// <param name="key">The key identifier</param>
+        /// <param name="value">Cache value</param>
+        public void Add(string key, object value)
+        {
+            var cacheItem = new CacheItem(DateTime.UtcNow, value);
+            _cache.AddOrUpdate(key, cacheItem, (key, val1) => cacheItem);
+        }
+
+        /// <summary>
+        /// Get a cached value
+        /// </summary>
+        /// <param name="key">The key identifier</param>
+        /// <param name="maxAge">The max age of the cached entry</param>
+        /// <returns>Cached value if it was in cache</returns>
+        public object? Get(string key, TimeSpan maxAge)
+        {
+            _cache.TryGetValue(key, out CacheItem value);
+            if (value == null)
+                return null;
+
+            if (DateTime.UtcNow - value.CacheTime > maxAge)
+            {
+                _cache.TryRemove(key, out _);
+                return null;
+            }
+
+            return value.Value;
+        }
+
+        private class CacheItem
+        {
+            public DateTime CacheTime { get; }
+            public object Value { get; }
+
+            public CacheItem(DateTime cacheTime, object value)
+            {
+                CacheTime = cacheTime;
+                Value = value;
+            }
+        }
+    }
+}