增加Mediat Pip特性，并用实例来做一个demo

Core/MAX.Core/Mediation/IValidatableRequest.cs

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+namespace MAX.Core.Mediation;
+
+/// <summary>提供请求自身的参数校验；不合法时抛出 ValidationException。</summary>
+public interface IValidatableRequest
+{
+    void Validate();
+}

Core/MAX.Core/Mediation/MediatRBehaviorAttribute.cs

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+// Adapted from ITIXO/MediatR.Extensions.AttributedBehaviors (MIT).
+// Copyright (c) 2021 ITIXO s.r.o. See ThirdPartyNotices/MediatR.Extensions.AttributedBehaviors.LICENSE.txt.
+using System.Runtime.CompilerServices;
+
+namespace MAX.Core.Mediation;
+
+/// <summary>声明命令管道。每个特性独占一行，按源码行号从上到下执行。</summary>
+[AttributeUsage(AttributeTargets.Class | AttributeTargets.Struct, AllowMultiple = true, Inherited = false)]
+public sealed class MediatRBehaviorAttribute(Type behaviorType, [CallerLineNumber] int lineNumber = 0) : Attribute
+{
+    public Type BehaviorType { get; } = behaviorType ?? throw new ArgumentNullException(nameof(behaviorType));
+    public int LineNumber { get; } = lineNumber;
+}