VS: Add support for .NET Standard and .NET Core

Currently, ALL_BUILD target forces use all generated C# projects to use .Net Framework. As discussed in the issue #20105 (closed), this PR:

1. Introduces DOTNET_TARGET_FRAMEWORK which takes precedence over DOTNET_TARGET_FRAMEWORK_VERSION.
2. For C# targets, sets msbuild property SkipGetTargetFrameworkProperties to true so that those targets can be built for .Net Core and .Net standard. Note that msbuild ignores SkipGetTargetFrameworkProperties and therefore, we don't set it for those projects.

Fixes: #20105 (closed)
Topic-rename: vs-dotnet-standard-core