Add `origins` cli subcommand

For our cors implementation, I'd like to automatically generate the list of allowed origins at build-time (we have staging/production environments, so we can't just hard-code, and would like for them to be kept fairly up to date):

It would be convenient if (similar to the `curls` command), there was an `origins` command (notice the lack of trailing path):

```
$ npx @ampproject/toolbox-cli@canary origins https://example.com
https://example-com.cdn.ampproject.org
https://example-com.amp.cloudflare.com
https://example-com.bing-amp.com
```

I'm happy to add it! But wanted to make an issue first to see if you all have any concerns/implementation preferences.

Start with the CLI's existing `curls` command and trace how its input and output are implemented. Add a matching `origins` command for a host URL, producing the origin list shown in the issue, and verify the command output with the project's existing CLI tests.

Written by the indexing model from the issue text.