Support sdk version branches/tags

[tmds]

The package guidelines for .NET Core describe a set of packages that are versioned based on the <runtime version> and <sdk version>.

To create these packages from source-build repo, we need branches/tags at the sdk level, for example: dotnet-sdk-2.1.3xx branch.

CC @dleeapho

[dseefeld]

We will add tags for each release which indicate which sdk version was released.

[tmds]

I see those tags are available. Thank you for providing them!