Enable plugin support in dotnet tool

[aetos382]

Is your feature request related to a problem?

Enable plugin support for dotnet tools.

Describe the solution you'd like

Some tools that work with NodeJS (e.g. webpack) support a number of plugins.
And npm allows to install both the CLI tool and plugins for the tool with npm install.
I think it would be useful to support a similar experience with the dotnet tool.

For example, I propose specification that dotnet-tools.json list the NuGet packages to be restored along with the tool, as follows.
The packages listed in additionalPackages need only be deployed so that the assemblies in them can be loaded from the tool, and the communication mechanism between the tool and the plugin would not need to be defined by the .NET SDK.

{
  "version": 2,
  "isRoot": true,
  "tools": {
    "microsoft.botsay": {
      "version": "2.0.0",
      "commands": [
        "botsay"
      ],
      "additionalPackages": [
        {
          "microsoft.botsay.plugin.XXX": {
            "version": "1.2.3"
          }
        },
        {
          // and other plugins
        }
      ]
    }
  }
}

[marcpopMSFT]

Old issue triage: draft PR about this that has no plans to follow up on NuGet/NuGet.Client#6544

[baronfel]

We also have an in-progress spec/design for a feature similar to this: dotnet/designs#337.