Add runtime for netstandard/netcore projects

[vlaldis]

Currently, the native windows binaries are not copied to bin folder for netstandard libs/netcore apps. Would be great to have it supported (usefull with FFmpeg.AutoGen)

[Ruslan-B]

I'll take a look, should be relatively easy to do.

[lukasjuhrich]

@Ruslan-B have you worked on this / can you revisit this? I am trying to achieve the same thing from a net48 project which might need some special casing; as far as I understand, it requires a FFmpeg.Native.build.targets file as a build asset with approximately these contents.