Add possibility to use a custom predefined container definitions repository for quick setup

Hi there,
First, thank you so much for the awesome work on this life-changing plugin!

A lot of companies need to have a custom docker base image layer (for security, standard on Linux distrib, reusability, and align with production deployment). In such a context, it's difficult to use the existing definitions from https://github.com/microsoft/vscode-dev-containers for development. The solution is then to create custom dev container definitions and regroup them in a repo, but it makes it a bit cumbersome to have to copy manually these ones each time you start a project or want to work with remote container.

Not sure how the current setup works, if its relay on an external service or directly from the git repo mentioning above, but it will be awesome if we could link from vscode a custom predefined repo and then the user would basically be able to navigate and list through the custom predefined dev docker containers definition as it is today for the one provided by Microsoft

Start by tracing how quick setup currently discovers and lists definitions from microsoft/vscode-dev-containers. Compare that flow with the requested custom repository configuration and determine how users would navigate and list its definitions in VS Code. Done means a configured repository can provide custom dev-container definitions through the same setup flow.