Add MicCheck.Client library with FeatureClient.IsEnabledAsync

Client-side SDK for checking feature flag state against the flags API,
with unit tests mocking HttpMessageHandler.

src/MicCheck.Client/Common/Guard.cs

@@ -0,0 +1,40 @@
+namespace MicCheck.Common;
+
+internal static class Guard
+{
+    public static void Null<T>(T t, string parameterName) where T : class
+    {
+        if (t is null)
+            throw new ArgumentNullException(parameterName, $"{nameof(parameterName)} can not be null");
+    }
+
+    public static void Empty(string value, string parameterName)
+    {
+        if (string.IsNullOrWhiteSpace(value))
+            throw new ArgumentException($"{parameterName} can not be empty", parameterName);
+    }
+
+    public static void Empty<T>(IEnumerable<T> collection, string parameterName)
+    {
+        if (collection == null || !collection.Any())
+            throw new ArgumentException($"{parameterName} can not be empty", parameterName);
+    }
+
+    public static void Negative(int value, string parameterName)
+    {
+        if (value < 0)
+            throw new ArgumentOutOfRangeException(parameterName, $"{parameterName} must be a positive number or zero");
+    }
+
+    public static void NegativeOrZero(int value, string parameterName)
+    {
+        if (value <= 0)
+            throw new ArgumentOutOfRangeException(parameterName, $"{nameof(parameterName)} must be a positive number greater then zero");
+    }
+
+    public static void Default<T>(T value, string parameterName)
+    {
+        if (EqualityComparer<T>.Default.Equals(value, default))
+            throw new ArgumentException($"{parameterName} can not be a default value", parameterName);
+    }
+}