Add example feature that uses `curl`

Writing a feature that uses `curl` is not exactly straight forward.

I copied some snippets from the [aws-cli](https://github.com/devcontainers/features/blob/baf47e22b0c3dc5b418ac57aae2e750d14bbc9a3/src/aws-cli/install.sh#L68-L87) feature. However, a well documented example in this repository would be awesome.

Specifically, some documentation explaining why installing `ca-certificates` manually is necessary. Forgetting to do so results in an error that is not so obvious.