Support devcontainer feature build/test/publish.

A large part of the ecosystem of devcontainers is the use of features to extend the environment, and potentially the IDE. Currently the scripts here do not support building, testing, or publishing devcontainer features, meaning either the feature must be baked into the base image (hindering reusability), or creating custom scripts to directly call the cli. This organization does have an additional repository that contains a GitHub action that can handle features, but unfortunately doesn't support azure at all.