Add dynamic-plugins-info plugin to the showcase image

### What do you want to improve?
Once [this PR](https://github.com/janus-idp/backstage-plugins/pull/1138) is merged the plugin will need to be referenced [here](https://github.com/janus-idp/backstage-showcase/blob/main/dynamic-plugins/imports/package.json) to include it into the backstage-showcase image.

Once that's done verify that the plugin installs and shows up in the UI correctly.

