Apply Strong-Name Signing to winrt.runtime.dll

[Scottj1s]

Based on .Net Core best practice, winrt.runtime.dll should be strong-name signed.

Modern frameworks can automatically apply binding redirects, so the burden of managing strict bindings is no longer an issue. And since strong-name signing is viral, winrt.runtime.dll must be strong-name signed to permit referencing from another strong-name signed assembly.

Points for consideration:

• Does it matter that there are two versions of winrt.runtime.dll (netstandard2 and net5)? Each would have a distinct hash. It's possible both could be loaded in a single process, in which case unique strong-name signatures would be required.
• Should winrt.runtime.dll be publicly signed, as .Net Core assemblies are? Or does true strong-name signing offer greater benefits, with only the requirement of distributing the private key?

[joperezr]

I maintain the https://github.com/dotnet/iot repo and currently trying to fix dotnet/iot#1091 which involves updating the way we call WinRT code to use CsWinRT instead. I'm currently blocked in doing so because of this issue as I am getting:

CSC : error CS8002: Referenced assembly 'Microsoft.Windows.SDK.NET, Version=10.0.18362.3, Culture=neutral, PublicKeyToken=null' does not have a strong name.

Without this repo producing strong-name signed assemblies, we are currently blocked with onboarding this. cc: @jkoritzinsky

[Scottj1s]

PR #420