Bug

Fixes: NuGet/Home#14305

Description

• Adds the initial dotnet package update command
  • Only adds it to NuGet.CommandLine.XPlat. The dotnet/sdk repo will need a change after this version of NuGet is inserted, in order to for the dotnet CLI to know that this command is available.
• As per the linked issue, it is intentionally minimal. Please see Add dotnet package update command Home#14304 for a list of sub-issues that are planned to improve the command
  • Only supports updating 1 package at a time
  • Only supports a single project, or a solution with a single project.
• Gets restore inputs via running msbuild as a child process. This is similar to what dotnet package add does, except package add does the child process in the dotnet/sdk repo, while package update does it with code in the NuGet.Client repo. However, all of this is (hopefully) temporary. A few weeks ago I added PackageSpecFactory and one of the intended goals was to enable easily creating the DGSpec in-process in dotnet CLI commands (as well as consolodating the existing 4 PackageSpec creation methods into 1). However, to save time, this code is doing the child process for now.
• Small changes to package add code so it's reusable for package update. That code is going to need heavy refactoring to make it support multiple packages and multiple projects, but it was quick to get this initial implementation done.

PR Checklist

• Meaningful title, helpful description and a linked NuGet/Home issue
• Added tests
• Link to an issue or pull request to update docs if this PR changes settings, environment variables, new feature, etc.