Add Crossgen pack for crossgen2 tool

[dagood]

For #906.

Requires support in the shared framework tooling: PR dotnet/arcade#4650.

I have this test build that produced the Crossgen pack by temporarily inlining the sharedfx tooling update: https://dev.azure.com/dnceng/internal/_build/results?buildId=487241

New test build for Crossgen2 pack: https://dev.azure.com/dnceng/internal/_build/results?buildId=488118&view=results

[dagood]

Filed tracking issues:

• Stop producing Crossgen2 transport package (use Crossgen2 pack downstream instead) #1869 "Stop producing Crossgen2 transport package (use Crossgen2 pack downstream instead)"
• Integrate Crossgen2 pack infrastructure into the shared framework tooling in Arcade #1867 "Integrate Crossgen2 pack infrastructure into the shared framework tooling in Arcade"

[dagood]

This is good to go, PTAL.

Here are some of the nupkgs from the test build (links need Microsoft auth)

• Microsoft.NETCore.App.Crossgen2.win-x64
• Microsoft.NETCore.App.Crossgen2.linux-x64
• Microsoft.NETCore.App.Crossgen2.linux-musl-x64

[fadimounir]

LGTM, but I'd like someone else more familiar with the build system to also sign off on the chagnes

[fadimounir]

Thanks for doing this work @dagood !

[dagood]

Test failures on runtime (CoreCLR Pri0 Test Run Windows_NT x64 checked) are a persistent known issue: https://github.com/dotnet/coreclr/issues/26241 (#702). I'll disregard and go ahead with the merge if approved.