Add ModelContextProtocol.Core package (#428)

src/ModelContextProtocol/IMcpServerBuilder.cs

@@ -0,0 +1,25 @@
+using ModelContextProtocol.Server;
+
+namespace Microsoft.Extensions.DependencyInjection;
+
+/// <summary>
+/// Provides a builder for configuring <see cref="IMcpServer"/> instances.
+/// </summary>
+/// <remarks>
+/// <para>
+/// The <see cref="IMcpServerBuilder"/> interface provides a fluent API for configuring Model Context Protocol (MCP) servers
+/// when using dependency injection. It exposes methods for registering tools, prompts, custom request handlers,
+/// and server transports, allowing for comprehensive server configuration through a chain of method calls.
+/// </para>
+/// <para>
+/// The builder is obtained from the <see cref="McpServerServiceCollectionExtensions.AddMcpServer"/> extension
+/// method and provides access to the underlying service collection via the <see cref="Services"/> property.
+/// </para>
+/// </remarks>
+public interface IMcpServerBuilder
+{
+    /// <summary>
+    /// Gets the associated service collection.
+    /// </summary>
+    IServiceCollection Services { get; }
+}