Add experimental MSBuild sdk project

[MarcoRossignoli]

This PR is adding the first version of the MSBuild sdk for MSTest, it could be interesting mainly for:

• add or remove dependencies without the needs to ask to the user to do it, we know that's not easy to flow new features that needs new dependency to the users, they've to read the documentation but it's hard to reach everyone.
• we can provide simple msbuild props knob to disable/enable features.
• we can have better "testing coverage" because we can always test "our" configurations and guarantee that everything is good, having less "moving parts", users will add dependencies only for their business code.
• it's battle tested and used by other well known .NET platform to simplify the "default usage" https://learn.microsoft.com/en-us/dotnet/core/project-sdk/overview#available-sdks

The outcome will be a project file like:

<Project Sdk="MSTest.Sdk/3.2.0-dev">
  <PropertyGroup>
    <TargetFramework>net8.0</TargetFramework>
    <!--<UseMSTestRunner>false</UseMSTestRunner>-->
    <!--<TestingPlatformDonetTestSupport>true</TestingPlatformDonetTestSupport>-->
  </PropertyGroup>
</Project>

The sample above is using the version inside the Sdk attribute self but it can be done "globally" using the global.json too like described here https://learn.microsoft.com/en-us/visualstudio/msbuild/how-to-use-project-sdk
Cons: the version needs to be coherent with the packages, it could be confusing have different custom versioning for the sdk.

[Evangelink]

I love it!!! That will be an awesome addon to MSTest and the best way for us to drive adding/prodiving more features to users.