Provide a feature that can install azure cli extensions

[asaikali]

I would like to be able to instal the azure cli and some extra extensions for example az extension add --name spring it would be nice to enhance the existing cli feature to set a list of extensions to install, or maybe add an azure cli extension feature that takes an array of extension names and installs them.

The blog post show to build such a feature. However, installing azure cli extensions so common it should be officially supported.

[jkeech]

@asaikali thanks for the recommendation! Would you or @pauldotyu (the author of the linked blog post) be willing to submit a PR to add an extensions option to the azure-cli feature in this repo?

Note that we are tracking an improvement in devcontainers/spec#91 which will make it easier for features to see which user is the remoteUser, so that you can easily run the extension installation command in the context of the correct user. It might be helpful to wait for that improvement to land first.