Add BeforeMicrosoftNETSdkTargets (#40344)

Add a hook similar to AfterMicrosoftNETSdkTargets, BeforeTargetFrameworkInferenceTargets, CustomBeforeMicrosoftCommonCrossTargetingTargets, etc.

This provides a unified entry point for custom MSBuild SDKs and MSBuild NuGet packages to execute just after the project file regardless of whether the build is cross targeting or not.

Author: baronfel

src/Tasks/Microsoft.NET.Build.Tasks/sdk/Sdk.targets

@@ -10,6 +10,7 @@ Copyright (c) .NET Foundation. All rights reserved.
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 -->
 <Project xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
+  <Import Project="$(BeforeMicrosoftNETSdkTargets)" Condition="'$(BeforeMicrosoftNETSdkTargets)' != ''"/>
 
   <!-- Using the same property as Microsoft.CSharp.targets and presumably Microsoft.VisualBasic.targets here -->
   <PropertyGroup Condition="'$(TargetFrameworks)' != '' and '$(TargetFramework)' == ''">