Refactor and enhance static ReCClient configuration

Introduced a new `BuildStaticClient(Action<StaticBuildConfiguration>)` method for flexible and detailed static `IServiceProvider` configuration. Added the `StaticBuildConfiguration` class to encapsulate optional settings like `BaseAddress`, `ConfigureClient`, `Logger`, and more.

Refactored existing `BuildStaticClient` overloads to use the new method, ensuring consistency and reducing duplication. Added support for optional `ILogger` instances and improved validation to enforce proper configuration.

Marked existing `BuildStaticClient` methods as obsolete, recommending the new method. Enhanced thread-safety using `Interlocked.CompareExchange`. Updated XML documentation and added conditional compilation for `NETFRAMEWORK` compatibility.

These changes improve maintainability, usability, and alignment with modern .NET practices.

src/ReC.Client/StaticBuildConfiguration.cs

@@ -0,0 +1,63 @@
+using Microsoft.Extensions.DependencyInjection;
+using Microsoft.Extensions.Logging;
+using System;
+using System.Net.Http;
+
+namespace ReC.Client
+{
+    /// <summary>
+    /// Configuration object for <see cref="ReCClient.BuildStaticClient(Action{StaticBuildConfiguration})"/>.
+    /// Groups all optional settings for the static <see cref="ReCClient"/> bootstrap path.
+    /// </summary>
+    /// <remarks>
+    /// Either <see cref="BaseAddress"/> or <see cref="ConfigureClient"/> must be set; setting both at the same time is not allowed.
+    /// </remarks>
+    public class StaticBuildConfiguration
+    {
+        /// <summary>
+        /// Base URI of the ReC API. Mutually exclusive with <see cref="ConfigureClient"/>.
+        /// </summary>
+#if NETFRAMEWORK
+        public string BaseAddress { get; set; }
+#else
+        public string? BaseAddress { get; set; }
+#endif
+
+        /// <summary>
+        /// Callback that configures the underlying <see cref="HttpClient"/>. Mutually exclusive with <see cref="BaseAddress"/>.
+        /// </summary>
+#if NETFRAMEWORK
+        public Action<HttpClient> ConfigureClient { get; set; }
+#else
+        public Action<HttpClient>? ConfigureClient { get; set; }
+#endif
+
+        /// <summary>
+        /// Optional callback to configure <see cref="ReCClientOptions"/>.
+        /// </summary>
+#if NETFRAMEWORK
+        public Action<ReCClientOptions> ConfigureOptions { get; set; }
+#else
+        public Action<ReCClientOptions>? ConfigureOptions { get; set; }
+#endif
+
+        /// <summary>
+        /// Optional logger instance to be registered as a singleton in the internal service collection.
+        /// </summary>
+#if NETFRAMEWORK
+        public ILogger Logger { get; set; }
+#else
+        public ILogger<ReCClient>? Logger { get; set; }
+#endif
+
+        /// <summary>
+        /// Optional callback for additional service registrations on the internal <see cref="IServiceCollection"/>
+        /// (e.g. <c>services.AddLogging(...)</c> or custom dependencies).
+        /// </summary>
+#if NETFRAMEWORK
+        public Action<IServiceCollection> ConfigureServices { get; set; }
+#else
+        public Action<IServiceCollection>? ConfigureServices { get; set; }
+#endif
+    }
+}