Improve CI checks

We should have automated CI checks for a pull request of a new extension, to confirm
- [ ] that a valid image version is available at the specified location
- [ ] that it includes the required dockerfile [metadata fields](https://docs.bluerobotics.com/ardusub-zola/software/onboard/BlueOS-1.1/extensions/#metadata-dockerfile)
- [ ] the validity of the URLs in the metadata
- [ ] that the extension runs (on a test system, I suppose?)
- [ ] the validity of the `register_service` endpoint (if there is one)
- [ ] old/deprecated metadata variables and structures aren't being used