Support commands to download targeting and runtime packs

[dsplaisted]

For .NET Core 3.0, we expect to separate targeting packs, which contain the assets needed at build time when targeting .NET Core 3.0 from runtime packs, which will include the platform-specific runtime components for .NET Core 3 which are deployed with self-contained apps.

Targeting packs will ship as part of the .NET Core SDK, and targeting packs and runtime packs will be acquired during restore as necessary via "download only package references".

In addition, we could also support explicit CLI commands to download targeting and runtime packs, something like the following:

• dotnet targeting-pack install netcoreapp3.0
• dotnet runtime-pack install Microsoft.NETCore.App -version 2.0.6 -rid linux-x64

These commands would basically download the relevant packages to the local NuGet cache. This could be useful for scenarios where you are going to go offline, or want to publish a self-contained app without restoring.

An alternative to these commands would be to build or publish a project targeting the desired framework / RID, so we currently don't think we need to add these commands. However, this issue tracks the idea.

[github-actions]

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