Implement package signature verification in Cake

> From https://github.com/NuGet/NuGetGallery/issues/8381#issuecomment-782458432
>
> _[As of .NET 5/NuGet 5.8](https://docs.microsoft.com/en-us/nuget/release-notes/nuget-5.8#summary-whats-new-in-58), NuGet has cross-platform support for package signature verification. This can bolster the security of package consumption, protecting against tampering and even enforcing strict client trust polices_.
>

Start by locating Cake’s NuGet package consumption path and any related tests; the issue names no files or entry points. Read the linked NuGet Gallery issue and NuGet 5.8 signature-verification notes to establish requirements. Done means Cake verifies package signatures cross-platform, with coverage for the supported trust policies.

Written by the indexing model from the issue text.