Add multi-targeting support to Microsoft.Azure.AppConfiguration.AspNetCore for .NET Core 3.1

[abhilasharora]

A large number of NuGet packages aren't produced for ASP.NET Core 3.1. The package Microsoft.Azure.AppConfiguration.AspNetCore has a dependency on the package Microsoft.AspNetCore.Http.Abstractions that is included in the list of obsolete packages.

This issue tracks the changes needed to add multi-targeting support for ASP.NET Core 3.1.

[abhilasharora]

These changes are now available in version 4.0.0-preview of the package.