Add feature for `dotnet-tool`

[Falco20019]

Right now, I have a couple dotnet tool install -g ${TOOL_NAME}@${TOOL_VERSION} calls that need to be run before a dotnet restore could be executed. Currently, I use the different lifecycle phases to ensure it's not done in parallel to the dotnet restore. But technically, it would be cleaner as feature which receives the name and optionally a version or additional flags like allow-roll-forward to ensure build order.

As local tools would need a manifest and usually would just need a dotnet tool restore, I only really see global tools as use-case here.

Could this be something interesting to the general or is it more like something only I'm interested in? Because if it's not, I will just go and do the feature on a user-based repository instead for personal usage. I could also offer an PR if there's interest in this at-all. Just making sure this would make sense to go into here 😀

[sliekens]

Hey Benjamin Krämer (@Falco20019), I thought a lot about adding this while improving the dotnet feature's install script, but there are a few problems:

1. The install script runs as root, but dotnet tool install -g installs tools into the user's home dir
   (in this case /root/.dotnet/tools)
   This is hard to implement reliably even with something like
   dotnet tool install --tool-path $_REMOTE_USER_HOME/.dotnet/tools
2. If your workspace uses a custom nuget.config, it is not available yet when the install script runs, and dotnet tool install might fail with package resolution errors for private tools
3. Even if 2) is somehow solved, dotnet tool install can require interactive authentication (for private package sources), while the script runs non-interactively

For my own projects, I've been running dotnet tool restore in a postStartCommand and this works relatively well.

You could always make custom, tool-specific local devcontainer features,
e.g. you can create a ./dotnet-outdated-tool/devcontainer-feature.json feature with pre-downloaded binaries.
Copying them into the right folder is still a bit hard, however.