Port StrongNamer to a .NET Standard MSBuild task.

[alexdrl]

This is a WIP that was created for testing the use of StrongNamer in the dotnet CLI.

This PR updates StrongNamer so it looks like this.

This depends on Mono.Cecil releasing a .NET Standard 2.0 version, with StrongNameKeyPair available.
Also, https://github.com/dotnet/corefx/issues/11225 will block the strong naming of this assemblies in Linux, which is our objective.

I opened up this PR also to see if there is some input and guidance, as our company needs strong naming, as DLLs need to be signed in an WPF application, and this DLLs are .NET Standard 2.0 libraries shared with an ASP.NET Core Application. Feel free to ask for changes, having this merged would be a win for our company.

PR TODOs:

• Update TestLibrary to work with the reference to the StrongNamer Target in the new path. (I don't know how to reference the Mono.Cecil assembly).

[ugumba]

I would really love to see a nuget for this!

[Sam13]

NuGet package would be nice 👍

[mariopasquali]

Hi, any plan to merge this? I am facing the need of using this from dotnet CLI and it would be greatly useful to just do a nuget update!

[e-llumin-build]

Hi @dsplaisted, would be great to see this PR included in the main nuget package if possible since Mono.Cecil appears to support .NET Standard 2.0 these days - i looked at the cecil nuspec and it has a netstatndard 2.0 target at least, thx for strong namer, this PR would make it truly cross platform afaik

[gohar94]

Any updates? Really need this to use strongnamer with the dotnet cli.

[clairernovotny]

This will hit some runtime issues PlatformNotSupportedException due to StrongNameKeyPair throwing on the PublicKey property on .NET Core.

I have this working and tested in my PR #41