Use ORAS .NET Library instead of ORAS CLI

[lbussell]

I just found that ORAS has a .NET library: https://github.com/oras-project/oras-dotnet

It is in the early stages of development, so the API surface is unstable. Once it is stable, we should use this instead of the CLI introduced in #1311.

Related:

• Produce signed .NET container images dotnet-docker#4589
• Add lifecycle annotations to images pushed to MCR #1201

[dotnet-issue-labeler]

I couldn't figure out the best area label to add to this issue. If you have write-permissions please help me learn by adding exactly one area label.

[lbussell]

[Triage] It would be good to get a handle on oras-dotnet's release timeline. Given the state of the current roadmap, it may not have the functionality we need for our use-cases yet.