Support for installing .NET Core shared frameworks

ASP.NET Core regularly needs to build against a shared framework which is not bundled by the .NET Core SDK downloads.

The way we did this in KoreBuild was to allow for an item group to define the additional shared frameworks required.

<!-- Installs Microsoft.NETCore.App -->
<DotNetCoreRuntime Include="1.0.5" />
<DotNetCoreRuntime Include="1.0.5" Feed="https://mydotnetclifeed/assets" FeedCredential="$(MySuperSecretToken)" />
<DotNetCoreRuntime Include="latest" Channel="1.0" />
<DotNetCoreRuntime Include="2.0.0" Arch="x64" InstallDir="C:\custom\" />

<!-- Installs the AspNetCore shared framework -->
<AspNetCoreRuntime Include="2.1.0-preview1-1234" Arch="arm64" />

During /t:Restore we ran a target which invoked dotnet-install.sh/ps1 to install the additional shared frameworks into DOTNET_ROOT.

We need this to convert from KoreBuild to Arcade. In the meantime, we’ve worked around this like this: https://github.com/aspnet/AspNetCore-Tooling/blob/03a21837eec155a2f810ee8e9645e984417439bd/eng/Tools.props#L3-L33

cc @rynowak @ryanbrandenburg @bricelam

[tmat]

It would imo make sense for these to be in global.json, since the version of dotnet is also there. Something like:

{
  "tools": {
    "dotnet": "2.1.400-preview-009088",
    "dotnet-shared-fx": [ "1.0.5", "2.1.0" ]
  },
}

[tmat]

I mean the workaround works, but would be nice to get first class support at some point.