Add VS Code Extension

[dhruvchand]

Is your feature request related to a problem? Please describe.
I would like to have my VS code extensions installed when I receive my Dev Box

Describe the solution you'd like
As a user, I should be able to:

1. specify a list of extensions (extension version optional) that ought to be installed on my Dev Box, using
2. extension id
3. path to file that contains export of code --list-extensions

setupTasks:

 - task: install-vscode-extension
    inputs:
      extension: "packageName@version "
      additionalOptions: 

For V1, we will NOT be supporting path to a JSON export of extensions from VS Code

 - task: install-vscode-extension
    inputs:
      extensions: "path/to/extension/list/export"
      additionalOptions: 

[jmatthiesen]

here's an issue submitted to the VSCode repo, which describes the challenges that some admins (e.g. School admins) are having installing VS Code extensions as part of base images: Allow extensions to be installed for all users · Issue #56614 · microsoft/vscode (github.com): microsoft/vscode#56614

Takeaway: to install VS Code extensions on Windows, you have to first download/install them as a user and then do extra work to manually copy them into a location for use in a base image.