.NET Core App 3.x targets require consumers add reference to Microsoft.Bcl.AsyncInterfaces

Currently, any .NET Core 3.x consumer must also manually add a reference to Microsoft.Bcl.AsyncInterfaces. This is because of the target to netcoreapp2.1 in the delivered NuGet package.

netcoreapp2.1 requires and consumes Microsoft.Bcl.AsyncInterfaces (transitively via System.Linq.Async). Therefore, the compiled netcoreapp2.1 binary references this DLL. The netstandard2.1 target doesn't include this requirement. However, NuGet considers the netcoreapp2.1 target to be a closer fit to netcoreapp3.x, and uses that instead of netstandard2.1.

This address this, we should also add a netcoreapp3.0 target.