C# - look for native library in nuget conventional locations

[dkackman]

.NET's package manage, NuGet, will install native libraries in a standard location, denoted by it's Runtime Identifier. This is the most common way for a .net package to be deployed.

This change modifies the C# POC code to look in those locations when loading the native library in the blst static constructor, making it easier to author NuGet packages that include the blst native libraries.