Manage .NET XPLAT cryptography

[PatoBeltran]

The dotnet APIs have specific XPLAT behaviors that we need to address when tackling signing and verification XPLAT. This will mean either document known behavior or do functional changes in specific platforms.

Some things are:

• X509RevocationMode.Offline is not supported on macOS
• OSCP are not supported in Linux

More info: https://github.com/dotnet/corefx/blob/master/Documentation/architecture/cross-platform-cryptography.md

Note: Make sure to read the document before implementing since it might change.

[clairernovotny]

I have a PR that shows the code changes needed to enable code signing for .NET Core: NuGet/NuGet.Client#2545

[Thraka]

Please note that when this gets added to NuGet, the .NET Core doc for dotnet restore needs to be updated. dotnet/docs#6496

[zkat]

This work is being superseded by the epic over at #7939, so I'm closing this issue.