Add DependencyInjection support for ReCClient

Introduced a `DependencyInjection` class in the `ReC.Client`
namespace to enable dependency injection for `ReCClient`.

Added an `AddRecClient` extension method for `IServiceCollection`
to register an `HttpClient` with a configurable `apiUri` as the
base address.

Included conditional `using System;` for .NET Framework
compatibility and added `Microsoft.Extensions.DependencyInjection`
to support DI services.

src/ReC.Client/DependencyInjection.cs

@@ -0,0 +1,18 @@
+using Microsoft.Extensions.DependencyInjection;
+#if NETFRAMEWORK
+using System;
+#endif
+
+namespace ReC.Client
+{
+    public static class DependencyInjection
+    {
+        public static IHttpClientBuilder AddRecClient(this IServiceCollection services, string apiUri)
+        {
+            return services.AddHttpClient(ReCClient.ClientName, client =>
+            {
+                client.BaseAddress = new Uri(apiUri);
+            });
+        }
+    }
+}