Add AddRecClient method and conditional using directive

Added a `using System.Net.Http;` directive within a `#if NETFRAMEWORK` block in `DependencyInjection.cs` to ensure compatibility with .NET Framework.

Introduced the `AddRecClient` extension method in the `DependencyInjection` class to enable custom configuration of `HttpClient` instances via an `Action<HttpClient>` delegate.

src/ReC.Client/DependencyInjection.cs

@@ -1,6 +1,7 @@
 using Microsoft.Extensions.DependencyInjection;
 #if NETFRAMEWORK
 using System;
+using System.Net.Http;
 #endif
 
 namespace ReC.Client
@@ -14,5 +15,10 @@ namespace ReC.Client
                 client.BaseAddress = new Uri(apiUri);
             });
         }
+
+        public static IHttpClientBuilder AddRecClient(this IServiceCollection services, Action<HttpClient> configureClient)
+        {
+            return services.AddHttpClient(ReCClient.ClientName, configureClient);
+        }
     }
 }