.NET Update Delay Could be Enhanced to Tie to Idle Time

Added in 3.0.0, https://github.com/dotnet/vscode-dotnet-runtime/pull/2420, was an update check that installs runtime updates and uninstalls out of date runtimes.

> An update check runs and tries to update the installed runtimes once daily. The update check runs after 5 minutes by default and is configurable in the new setting: runtimeUpdateDelaySeconds. Users could already disable the install mechanism, so this is not a 'new' power they have.

@/drewnoakes had a great suggestion - the idea of modifying that duration and making it happen when the user is idle. I think we could tie that `runtimeUpdateDelaySeconds` to some inactivity check. `onDidChangeWindowState` doesn't necessarily mean the user is idle, but `onDidChangeTextDocument` + that may work as a sufficient way to make the update less disruptive. The 5 minute default period was relatively arbitrary I will admit, though I think it was somewhat influenced by a discussion I had with CDK about the telemetry and how often the rest of startup takes in the worst case.

The impact for the update and uninstalls won't be negligible but it also doesn't block anything, it just takes up some of the active thread. I think vscode does a decent job at load balancing the extensions but haven't looked at that code precisely.