Enable PackageValidation tool

[Marusyk]

The SDK provides a tool to validate NuGet packages right after creating them. At the moment, it provides the following checks:

• Validates that there are no breaking changes across versions
• Validates that the package has the same set of publics APIs for all the different runtime-specific implementations
• Helps developers catch any applicability holes

To enable it, you can add the following property to your project file:

<Project>
  <PropertyGroup>
    <EnablePackageValidation>true</EnablePackageValidation>

    <!-- Optional: Detect breaking changes from a previous stable version -->
    <PackageValidationBaselineVersion>1.0.0</PackageValidationBaselineVersion>
  </PropertyGroup>
</Project>