Support ProjectServicingConfiguration build skips in sharedfx tooling

[dagood]

Support configuring which projects build for a certain release via ProjectServicingConfiguration items in eng/Versions.props:

  <!--
    Servicing build settings.
    * To enable a package build for the current patch release, set PatchVersion to match the current
      patch version of that package. (major.minor.patch)
    * Do not delete these lines to disable the package build. When PatchVersion is incremented at
      the beginning of the next servicing release, the package automatically stops building because
      the version no longer matches.
    * These items also keep track of the last time each package was patched, enabling source-build
      to produce the correct old version number using current sources.
  -->
  <ItemGroup Condition="'$(StabilizePackageVersion)' == 'true'">
    <ProjectServicingConfiguration Include="Microsoft.NETCore.App.Ref" PatchVersion="0" />
    <ProjectServicingConfiguration Include="NETStandard.Library.Ref" PatchVersion="0" />
  </ItemGroup>

Full context of the repo-side config: dotnet/core-setup@master...dagood:servicing-skips

This adds support for https://github.com/dotnet/core-setup/issues/8735 and https://github.com/dotnet/core-setup/issues/8507 to the shared tooling.

Behavior (did a few stabilized local builds with the values tweaked):

NETCoreApp Runtime	NETCoreApp Ref	NETStandard Ref
5.0.0	5.0.0	N/A
5.0.1	N/A	N/A
5.0.0 (source-build)	5.0.0	2.1.0
5.0.1 (source-build)	5.0.0	2.1.0

I ported the changes to Core-Setup release/3.0, and the results are the same there (and they include WindowsDesktop with the same pattern as NETCoreApp).

/cc @nguerrera @mmitche @Anipik