Allow custom package ID or support RID-specific Framework Packs in Microsoft.DotNet.Build.Tasks.Packaging

https://github.com/dotnet/arcade/blob/a1805103791e43031355e11c0d037bca803a9593/src/Microsoft.DotNet.Build.Tasks.Packaging/src/build/Packaging.targets#L34-L43

I had to work around this to create `Microsoft..App.Runtime.` in Core-Setup, I set global properties in an `MSBuild` task to prevent my values from being overridden. That seemed like the most reasonable way to get it done because there are a ton of properties that derive their values from `Id` statically.

(Technically I worked around the BuildTools version of this package, but it appears to be the same here.)

Pack background: https://github.com/dotnet/designs/pull/50