KoreBuild as a solution SDK

[natemcmaster]

Vision: make KoreBuild a "solution" SDK.

What KoreBuild basically does now is extend the standard lifecycle of solution targets like Build, Pack, Clean. When improvements to the MSBuild SDK acquisition experience land, we could make KoreBuild part of the solution build.

Usage:

+MyRepo.sln
+before.MyRepo.sln.targets

<!-- before.MyRepo.sln.targets -->
<Project Sdk="KoreBuild.Sdk/1.0.0">
</Project>

msbuild /t:KoreBuild MyRepo.sln

Problem it may solve:

• Tasks from BuildTools can get out of sync with targets in KoreBuild
• Make it easier to pin for branching for release, dev, pinning KoreBuild to the right version currently requires
• Make it easier to use side-by-side versions of KoreBuild in Universe builds.
• Make the acquisition experience cleaner. (Relies on MSBuild 15 Sdk Design dotnet/msbuild#1493)

[natemcmaster]

We don't really need this now that we are doing #258. Closing as this is redundant.